What appears to be an Air Intake? Any comments

I do a lot one one year warranty inspections and ran across this today. The house has two split systems (condensing coils on the exterior with air handlers/evaporative coils in the attic). The one system had, what I would call a normal Air Intake/Return in the Ceiling (Grill that opens and contains a filter). However, the other system, same manufacturer just a different size, had a cover that can be removed and contained the filter. Seemed to be a return. I was unable to locate anything else that would be considered an Air Return. I have never run across this. I did recommend a HVAC technician to further evaluate and make repairs as needed.

What’s inside the plenum?
I see a take-off on the right side of the filter, is this a pass-through distribution box with a drop down filter access door?

I could not find any access door beyond this point. The bedrooms had “Jumper” vents inside the rooms, but I wasn’t able to locate any jumper vents in this hallway either making me think they this box may have been for that purpose, they just ducted it wrong and left out a return for this unit. (I call them Jumper vents, basically vents from the inside of the room to the outside to prevent the door from slamming shut when the AC kicks on). There was definitely air being sucked up thru this access door.

Do you have photos of the air handlers in the attic?

I see all this return duct and plenum material here. Are these two units joined on each side of this plenum? Maybe they share. Hard to tell from the photos. I have seen two systems sharing the same return plenum in my travels.

Bryan, in the top photo there’s apparently a significant space in that plenum or whatever it is between the bottom of the filter and the door when the door’s closed. If that’s the case, that filter is useless and repair is required.