It might possibly be condensation or something weird like that. It's probably not since it sounds like the pool is large but the only other thing I can think of is either a rear windshield washer leak (but you said the carpet is not wet) or a fule leak (since it is right back there) but you would obviously know that it was fuel. Maybe a hole in the bottom or side of the tire compartment??? Has it rained where you are at?? Sorry I can't be much help. That's a very strange problem.