A similar thing happened to my wifes car two years ago at a body shop. When they returned the car to us in January we would have never thought to check the a/c. When the summer rolled around the air conditioning was not working. They claimed that the freon was lost when the car was involved in the accident (it was a front on hit). I pulled a vacuum on the system and it held, there was no leak. We notified the insurance company about the scam. I had already charged the system by the time the insurance company got back to us. You have to keeps the wife's car runnung right.
I agree you have to be careful, there are a lot of crooks out there when it comes to r12