As far as drums go, a lot of what is called 'flood/water damage' to the interior of drums is actually from condensation. If both heads are non-ported, you get a build-up of relatively humid air in the drum (via the grommet hole). You always see bassdrums with the water stains/damage in the bottom of the interior. The condensation in a standing bassdrum will collect at the bottom. You see that a lot on older bassdrums as bass drums were almost never ported until the late 60s.. You leave a warm humid environment (e.g. packed stuffy club chock full of respirating humans, your basement) and put the drums into a cold environment (your car in winter, the garage). The moist air inside the drum reaches the dewpoint, and the water condenses on the interior of the drum (stains, cracks, crusty, rusty internal screws etc..).
Conversely, you take your drums from a chilled/cold environment or from outside in winter (a cold car, garage, air conditioned environment) into a warm humid environment (a packed club, a studio or an outside summer gig), water condenses on the relatively colder object. In this instance, the exterior of the drum will mostly be affected, but here too the warm moister air finds its way in through the grommet hole and moisture condenses on the relatively colder, poorly ventilated interior. In this case as you it the drum during the gig, the vacuum created by hitting the drum pushes the cooler air out and pulls the moist warmer air in. At the end of the show any moisture has a hard time exiting as your not playing anymore/forcing the air in and out via the grommet hole.
When I leave a hot humid club gig in winter I'll get the drums home & press the heads for a minute or so to breathe air (less humid) back into the drums. Also in the summer if you play a gig in the hot humid weather do the same if you bring them right back to your air-conditioned house..
Cymbals & hardware too; When you get home leave your bag unzipped to let air flow through.
*It also applies to electric gear too! If you have an amp head/PA in your car in winter for a few hours (or any relatively colder environment) and take it into a warm humid club, studio, house, water can/will condense on the electronics..