Okay, since my buddy, Oxoroniox (man, you need a better username ) isn't completely convinced about the pressure side leak, I'm going to post my "pie-in-the-sky" scenario about it being in the skimmer.

I can envision a crack in the skimmer running diagonally to the water flow and very near a point where the water is forced to change direction. With no flow through the skimmer, the sides of the crack stay virtually together and there is almost no leak.

When the pump comes on, however, there is enough pressure from the redirection of the water that the thin plastic sides gap open a little allowing quite a large volume of water to escape. If this is feasible, I do not think this scenario would intake air.

Okay, Okay, I'll start drinking less from now on