Mazzy,
Not daft - I also do that. I fly a 172, and have never drained more than a couple of drops of water out. It's just at the club the other day someone was speculating on the reason for an engine failure in a recent accident, and suggested that if you did have water in the tanks, and it froze over night, you could drain the tanks, set off happy in the knoweldge you've got all the water out of the system, and then run into trouble as the ice melts...
I'll check the filler cap seals as suggested, and keep the tanks topped off each night.
DW