Engine replacement

Had engine replaced May 2017. Now have a small oil leak and when I did oil change last week, guy said there was no oil basically. He said to take it back to other shop that replaced motor. I was watching them and they were pushing up on something from under the car. I went out and asked and they said the bolts from the motor mounts were missing? Only two out of eight were there. I asked if they had touched it during the motor replacement and was told no. They said they must have fallen out? Or someone was trying to steal the catalytic converter and took them out. Is this possible? Thank you for any help! They did replace the bolts/screws for me and said that this should fix the oil leak problem.