Well if it's happened to you and I maybe it's an "industry standard practice". I wouldn't be surprised somewhere buried deep in the fine print of laws that after X number of years they can sweep the money back into their coffers permanently. It's not like they don't know your address to send you a check or anything.
I think it must be industry practice since it has happened to me 3 times, the other 2 times being at GP offices.
Before we ER'ed we had dental insurance and the dentist's office was very good at sending out regular detailed summaries of charges, money paid by the insurance and money paid by me. I didn't mind the few occasions I was in credit 'cos they were usually the ones owed money, not me, and we did make regular visits.