26sonic

I have a 02 31 sonic with twin 496 ho engines. Ikeep finding water in the bilge after using boat for the day and letting it sit over nite. I have ruled out any leaks from out side of boat because it has new bellows etc. . if I suck out any water in bilge with shop vacuum and not run engines the bilge is bone dry the next day. but if I run the engines for half hr or so and look in bilge there is no sign of a obvious leak? if I come down the next day and check the bilge will have 1-2 gallons of water? I can run this experiment over and over with same results any ideas where this leak could be coming from in the closed water system or is it something else?