I've circled them in blue. Obviously the open one is the piping to the skimmer sunction, but what is the other one for? It is capped off and has been caulked around. Should it be? I noticed the caulking is a little thin on 2 of the sides, could this cause a suction side air leak (I have a small air leak somewhere that is causing some bubbles to come out my 3 pool return jets, haven't been able to figure out where though). Should I put some new caulking around it? Also, what's the flap for?