Missing wheel cap after car went for service.

I sent a car for service with my husband and the car came back with a missing wheel cap. I then reported it and the manager and the responsible technician argued that the car came in like that.. I asked for a check list which they could not provide since they had already done the file apparently. The next day they showed me a document with a diagram that showed a missing cap on the left meanwhile my car is missing a cap on the right not on left. The one on the left is the exact same one since it has mark that I know. They then argued that they could have swapped the caps when returning them but that's not the case because the one on the right is missing and is not anywhere on the car. The amazing thing is someome made initials on the document next to the diagram which they claim is my husband but he doesn't recognize the initials. Someone inside initialled. Question is who edited the diagram and initialled. I am so disappointed for such a service that I paid for. Now I must go and buy wheel caps.