I have one in my kitchen that has been tripping for a few months. The only thing plugged into the outlet is our microwave, and the GCFI usually flips when we start the microwave or, strangely enough, when we open the door to the microwave (perhaps because the light comes on). I wish I would have taken the time to figure this out before, but we usually just reset it and keep going.
Does this suggest that the circuit may be overloaded (as opposed to there being moisture somewhere)? I think our fridge may be on that same circuit. I wonder what else . . .