I recall an instance where a man was in an accident, not really injured, but decided to ride the ambulance to the hospital to be checked out. He voluntarily surrendered his firearm to the officer who was working the accident for safekeeping. When he got out of the hospital and tried to get his gun back from the department, he found that they had not only run a serial number check on the gun, but also fired it to add to a database of ballistic fingerprints. They also refused to turn the gun back over to him for a period of time, I think they were waiting for the results of the gun trace.
Anyone remember the actual case I am talking about? it was fairly recently.
I also know of a few instances where the police either destroyed or refused to return ammunition that was with a gun that they took possession of, regardless of whether the gun was linked with a crime.