In the 2nd floor bathroom (sink, toilet, tub), I noticed a flexible hose coming into the sink drain. It's below the counter, above the trap, very similar to a dishwasher draining into a disposal. There are no other plumbing fixtures in the second floor, and from what I can see the flexible hose connects to standard pvc pipe at the wall behind the cabinet. I don't know where it goes after that.
I do not see this on any other sink drains in the house. Is it for venting? Or something else?