That settles the matter then as your oil sump must be completely fine, I had an oil leakage when parked and that was due to damage on the sump.
Apart from that it could be anything from a tappet cover leak, incorrect fitting when axle was changed or many more different reasons. If it is leaking a lot then its best to tow and have it looked at by the same mechanic. That will be best since if it started leaking after him working on it then chances are he messed up.