It normaly happens on cars with big silencers, nothing to be worried about. Some silencers have a drain hole. Just drive it and as it warms up the moisture will evaporate. HTH
Burning petrol produces water. Lots of it.
When the exhaust is hot the water will pass straight through as steam, but whan the exhaust is cold the warter vapour will condense and get blown out of the tailipe by the exhaust gasses, along with particles of soot.