Probably after effects of wet sumping or could be excessively breathing engine (piston rings) that's sending a fine mist into the throttle body/air filter area continuously - when you stop it slowly accumulates to bottom of filter/throttle body and seeps out onto cam chest cover over a couple of days then down onto floor.
Fit a remote catch tank/breather tank which should solve the issue if your that concerned.
Check the fuel hoses as a priority of course but looking at the picture that wouldn't be petrol staining the ground like that.
