Leaking coolant with the truck off?

[Number21]

The last few days I have come out to find a small puddle under my truck and what looks like a leaking thermostat gasket. I dried up all the coolant on the engine to look for the leak, but, it wasn't leaking anymore.

Just found another puddle under my truck, I haven't even driven anywhere today! It was dry when I parked it. Again I dried it up and started the motor but can't see any leaks. I just replaced the t-stat and the gasket not long ago.

Anybody have any idea what's going on here? I'd swear it's coming from the t-stat gasket, but only when the truck is parked?!