I have a bathroom on the 2nd floor with a toilet, sink, and bathtub. The toilet in that bath recently had a growth of flies due to the water in the bowl dropping below the trap level and exposing the bathroom to sewer smell and flies. The flies have since been killed, but the water in the toilet slowly drains out of the bowl (about 1/8-1/4 inch a week). This bathroom is only used when we have guests. I understand that this could be happenign becaus eof a vacuum created with the vent pipe. I went in my attic and ahve verified that this bathroom has its own vent pipe. But strangly enough I have three vent pipes for different areas. The one for the master is 1 1/2″ pipe, the one for the washer and 1/2 bath downstairs is a 2″ pipe, and the one for the guest bath is a 3″ pipe. Could the size of the pipe be affecting the toilet? Is there something else I am missing? SHould I be concerned with the little amount of water that disappears each week?