Spilt petrol goes a long way, as it is very viscous(thin), so a small leak can look enormous, fuel does expand in heat, if you filled to the brim then that is the problem, NEVER ever on a cossie fill to the brim, just to the first cut off.
There is a small hole in the cap for vent, as well as a pipe that runs around the side of the tank for venting in use, it's likely to have come out the cap itself, then there is a hole in the bottom of the "dish" area around the filler/cap for it to leak through(meant for water).
tabetha