Never seen one of these before. It is hooked to the drain and has hot water running through it. Has anyone see this before?
Circulator pump.
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As already mentioned, a circulator pump, it’s purpose is to keep hot water flowing through the hot water distribution pipes, thie reaso is so when you turn on the hot water at any fixture in the house, you won’t have to wait for the water to get hot.
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Are you doing home inspections?
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Circulator pump.
Better for the pipes if it has a return temperature sensor, and does not run 2/4/7.
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There was a day when I first saw one. That was a long time ago and they were a rarity. As a Plumbing Inspector I see them all the time in larger homes. Rarely in smaller ones.
90% of the ones I see are unplugged… always seems strange as it would be a nice feature. A house doesn’t even need to be that big for it to be a pretty big PITA for how long it takes to get hot water at some far away fixtures.
Hot water Recirculating pump have a very mixed reputation. From increased energy costs to pipe ware and leaks.
I didn’t know about this one, interesting.
Yeah, gotta be careful that the water moving through the pipes will “ware” the piping down until the whole house needs to be re-plumbed!! 
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That’s because it’s not true.
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Not sure if you were being sarcastic or what, but… in case, you cared.
Are you familiar with hydronic space heating and the concept of erosion corrosion or pipe scrubbing? if not installed correctly (the circulator, piping, etc…), it’s absolutely possible to put excessive wear on the pipes. Yes, it doesn’t happen over night, but it can definitely cause issues prematurely. But hey, in 45 years of plumbing, in 10 states, and after 50,000 installations, never had a problem 
If you care to learn, here is a starting point, then more reading can be done. Too many plumbers I asked about this are completely ignored of the fact. Most modern tradesman are not eager to learn, they just want to get it done, get paid, and go home. Yes, it will last some, then they are nowhere to be found.
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I concur!
Best analogy yet!
You got me, I was being sarcastic.
Circulation pumps are a standard element of many if not most commercial distribution systems, sometimes used in residential applications too. Even though the system may fail, or not work properly, the pipe failure is not something that is expected or a concer.
That was pretty good article, thanks.
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Not if it’s on demand.
No pipe ware.
If you’re not careful, you get a hole house circulation system.
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More like 40% for me. They are often on timers and I suspect some people don’t understand the timer or the concept or both.
Mine is plugged in and it works great.