was this smell noticablle immediatelly after the work was done or is it recent.
could be as simple as hair and such caught up in the p-trap. I have once found a problem like this caused by a hangar underneith ( galvanized strapping ) not done well at all ( a nail placed through the loop and not a stove bolt ) that released and caused the line to bow and in effect created an s trap that partially syphoned the trap after use. problem was resolved by properly supporting the pipe and checking grade. though this was a very odd instance and unlikely in many cases, my intent is to say that there are a number of things, some subtle that can cause issues.