I agree with Tardis. Most of the fluids in your car including tranny fluid, coolant, power steering fluid, and even fuel, are pressurized when your engine is running and not when it's parked and cool. A leak to any of these could easily be small enough to be only significant when the engine is on and leave a trace when the engine is first turned off as it depressurizes. Your brake fluid is only pressurized when you press the pedal.
Whatever the fluid is, it's imperative that you (1) check all your fluid levels, and (2) get this checked out. Some leaks, like minor oil leaks, are minor and you can live with them if you monitor your fluid level. Some leaks, like tranny fluid leaks and coolant leaks, can result in very expensive major repairs if left unaddressed. Some, like brake fluid leaks and fuel leaks, can kill you. You need to find out which yours is.
Get this checked out ASAP. Best case, you'll find out it's nothing important. It might even be condensation from your AC. Likely case, you'll find you need some repair work. Worst case, it'll save your life.