Mystery Smell

2016 Mazda 3

My husband had his car in the shop yesterday and they only worked on oil/rotations/general maintenance, but after he got home we noticed the garage smelled like fuel or oil. We let it air out and are trying to find the source of the smell. Nothing dripping, gas cap is on, under the hood no leaks are visible, and the oil cap is on. What else could we look for, and how would we know if it is safe to start to return to the shop?