I have had this exact problem in the 172 I fly. The A&P that checked it out said that it was caused by the tank being left completely full and temperature changes causing the fuel feeder gasket to seep fuel where it's mated to the tank. Just don't leave it sitting out topped-off or have them fill it to about an inch below top and it should take care of it.
Clean the area off and don't top it off until you're about to fly, and I bet you won't see it anymore. Granted that it's something which should get fixed at the next service interval, but it's probably not a critical safety issue. (at least in my case it wasn't) Still have it looked at first though.
