Just an update to my problem. Upon closer inspection it appears that the previous owner either drove off with the gas pump nozzle or the Hulk was pumping gas causing damage to the filler spout. That put a small crease on the filler neck which did not allow the gas cap to seal well. My mechanic used a brass punch to reset the crease, as well he added an additional rubber gasket to the gas cap just in case. The problems seems to be fixed.
1980 BMW 528i