I took the OP at his word that there is filler although I also can't see it in the pics posted. I agree ants that it appears to be a parking lot impact from some time ago (rust), but the OP can see/feel/magnet and I can't.

If there is no evidence of a repair, then it is possible it happened at the mechanic's but more probable that it happened at a shopping center. In that case, don't hassle a good mechanic over it.