I am not totally unfamiliar with plumbing, but this one has me stumped. I flush the toilet before I go to bed. In the morning the water level in the bowl has dropped a couple of inches. When the toilet is flushed there appears to be an air bubble which comes up, but the toilet flushes almost fully. The next flushes are fine and as long as the toilet doesn't sit without use it appears to work normally. I have gone out for the entire day and upon return the bowl level is low and the problem occurs.

I'm not sure where the water is going. Is it possible there is a minor clog in the trap which causes it to lose suction and some water goes down the drain after a couple of hours? Is it possible since we're in a condo that there is a big pull on the main causing this water loss. I see no loss of water around the base of the toilet and my unit is right above the lobby and the lobby ceiling looks dry. I'd suspect a loss if it weren't for the bubbling up. Thank you for your help.