Might help to edit the post above to keep things on track.
Done.
I don’t believe 300.22(B) applies to this receptacle - it’s not really “wiring” and it’s not really running “trough” a plenum. What do you think?
On a few older homes that originally did not have HVAC, I have found receptacles in the plenum area for running condensate pumps as well. It always looked like Fido’s behind but worked for the intended purpose.
I do not see the receptacles shown as being in a plenum, unless it was a plenum ceiling.
No, it would be more accurately referred to as a “duct,” which would fall under the same rules.
Jeff,
It would be my humble opinion the receptacle in the duct would not be allowed. The materials used for the installation are not allowed. The receptacle is not currently being used for an allowed use. While one could argue it is at the opening in the duct, it does still reside inside the duct and therefore the previous stated code would apply.
I believe the code in this case is limit the duct from spreading toxic smoke and fire to other area’s.
What was it for?
I can only assume that it was for an electric air filter, as Mike previously mentioned.
I agree that it is not allowed in this instance, because there is no equipment being served.
NEC 300.22(B):
Equipment and devices shall be permitted within such ducts
or plenum chambers only if necessary for their direct action
upon, or sensing of, the contained air.
Some people will do anything I had to cawl 50 yards just to see that mess or it seem like 50 yard by the time I got back to the entry hole
Keeps you in shape Charley. :)
At least you caught it Charley and that is what we all strive to do. At least the ones that really care about the safety of the Client.
I agree Frank, and since the equipment had been removed, the receptacle should no longer be there either.
That would be my thinking as well (or interpretation of the code), it is no longer necessary.
Generally I offer no advice, information or ask for any in these threads (read only) but I just happened to be studying this section at the time I saw your post.
If the receptacle is active, it is wired,so the question is answered. The reason you wouldn’t want it where it is located in the picture, is self evident. All of that debris in the holes in the receptacle can’t be good! 
http://images2.cableorganizer.com/articles/images/Cat6-Plenum.jpgThe plenum is an open space found above a building’s dropped ceilings, and it is often used for air circulation in heating and air-conditioning systems. In addition to being utilized for routing air, plenum spaces often house telephone and network communication cables which are being run from one location to another. Any cables which are being routed through above-ceiling air circulation spaces are therefore known as plenum cables, and need to adhere to special fire-safety standards.
According to Article 800 of the National Electrical Code (NEC), plenum cables must comply with the specifications for flammability and smoke density outlined in Underwriters Laboratories (UL) and National Fire Protection Association (NFPA) testing methods.
Plenum-rated cables are distinguished from other types of cabling by the special types of plastics used in their jackets. Because plenum cables are routed through air circulation spaces which contain very few fire barriers, they need to be coated in flame-retardant, low smoke materials such as polyvinyl chloride (PVC), fluorinated ethylene polymer (FEP), or a polyolefin. These plastics offer good resistance against fire, and in the event that they do begin to burn, they will not emit large quantities of harmful fumes.
If your cabling application requires materials that are flame-retardant or compliant with strict safety standards, opt for plenum-rated products. CableOrganizer.com offers a full line of plenum-grade cable, raceway, and wire connectors to meet all of your fire safety requirements!
http://www.cableorganizer.com/articles/plenum-cable.htm
My interpetation would be that the receptacle is not Plenum rated as well as the power cord of whatever plugged into it.
Any duct equipment sensor only should be installed in the plenum.
That is my take on it based on Commercial Building past experience.
Maybe it has changed. 
Air plenums above ceilings and constructed plenums(duckwork) are not the same thing.
Here is a helpful video from Mike Holt explaining the issue.
Plenums or other air-handling spaces
Because many of today’s buildings use the space above a suspended ceiling to handle environmental air, the first — and quite possibly the most important — step in the process is to confirm if the HVAC systems will use this type of air distribution or if the building will be fully ducted. When the space above the ceiling is used for environmental air handling, building codes are usually more stringent about what may or may not be installed in the space. These requirements are necessary due to the potential for spreading smoke and/or other toxic gases to other areas of the building via the HVAC system in the event of a fire.
As always, you should refer to the local authority having jurisdiction (AHJ) to determine all applicable codes. In most cases, however, there are two main code references that dictate what you may install in these particular areas: the National Electrical Code (NEC), specifically Sec. 300.22© – “Other Space Used for Environmental Air,” and the International Mechanical Code (IMC), Sec. 602 – “Plenums.” Generally speaking, if not enclosed in an appropriate metallic or non-combustible enclosure, all electronic equipment wiring must be tested and listed in accordance with stringent UL standards in order to be installed in a plenum or air-handling space.
http://ecmweb.com/voice_data_video/103ecmVDVpic2.gif
Unfortunately, it’s not uncommon to find violations of these code requirements in a newly constructed building, many of which include the supporting hardware and cabling for low-voltage systems, such as AV systems, security systems, and voice/data systems. For example, Photo 1 reveals a common violation with the installation of IP security cameras, AV projectors, and other IT equipment, such as wireless access points. These devices often include the use of proprietary cable connections (or whips) and/or the installation of cabling, jacks, modular plugs, faceplates, and outlet boxes to provide the data connections required as part of the system. Unless all of these supporting items are appropriately listed and labeled for installation in a plenum or other air-handling space, you must install them within an appropriate non-combustible enclosure. Another interesting point to note in Photo 1 is the fact that the installer left some things behind in the ceiling space upon completion of his work. Notice the discarded product packaging in the back right corner of this image, and the cardboard used to shim the supports for the camera. Not only does this situation violate code rules, but it also unnecessarily increases the potential fire load. Therefore, it should not be used, regardless of the type of air distribution system.
One commonly missed accessory in AV installations, such as sound reinforcement, public address, or mass notification systems, are the back-cans for ceiling-mounted speakers (Photo 2). These devices, which are metallic (or non-combustible) enclosures surrounding the speaker and internal components, are required when the speaker would otherwise be exposed to the air-handling space above the ceiling and has not been listed as suitable for air-handling spaces.
One of the more significant areas of concern with low-voltage devices pertains to the cabling used to interconnect them. Commonly, these cables are not required to be run in conduit (except in some jurisdictions like Chicago), but they are required to be properly supported and listed for the environment they are installed in. Most of the time, the desire to reduce costs and an increased ease of maintenance will lead to the elimination of most of the conduit used for these cables in favor of j-hooks, cable trays, or other open air hangers.
Same link I posted in 2nd post of the thread.
Air Plenum
Any space used to convey air in a building, furnace, or structure. The space above a suspended ceiling is often used as an air plenum.
A condition, space, or enclosure in which air or other gas is at a pressure greater than that of the outside atmosphere.
Read more: Answers - The Most Trusted Place for Answering Life's Questions
Plenum is an enclosed portion of the building structure that is designed to allow air movement, and thereby serve as part of an air distribution system. Supply, return, exhaust, relief and ventilation air plenums shall be limited to uninhabited crawl spaces, areas above a ceiling or below the floor, attic spaces and mechanical equipment rooms.
Plenum. Air compartment or chamber to which one or more ducts are connected and that forms part of the air distribution system.
Sorry for the repeat Marcel.
I think the change to environmental air space make it more clear.
I ran a lot of plenum rated cable back in the day.
I didn’t run the cable Mike, but was sure as hell responsible as to what went inside the plenums to stay within Code requirements as a builder.
I did one job, where exposed kraft faced insulation was exposed in the plenum under renovation. Wasn’t a plenum before, but was under the renovation.
We got paid to take it down and one worker went on workers Comp. for inhaling dust. Dust mask was not enough.