I never really thought about why there were two pumps back there but now I'm wondering if this is a booster pump. When it turns on, separately, it pushes out its own return, although the pump appears to be hooked to the filtration system, in the middle of the pool that an automatic pool cleaner used to be hooked up to. I thought was identified as a Polaris 360 when I took it in once, but maybe could have been a 280 or 380 if this is a booster. I'm looking to buy a new one soon so I'm glad I noticed this other pump now!
One more question: what is the hose connection doing on the filtration system? Air bleed?
The second pump I'm referring to is at the bottom of the picture vertically mounted. The filter pump is mounted horizontally right next to the filter near the top middle of the pic.
It turns on with a completely separate timer here: