I just read this in Austrian Times. It happened in Vienna, but it could happen anywhere.
Two Romanians claiming to be plain-clothed police officers have been arrested for stealing from tourists yesterday (Sun).
Daniel D., 42, and Valerica D., 40, wore metal badges saying "Police Officer" and asked holidaymakers in the centre of Vienna for their IDs. The duo then nicked their wallets and mobile phones.
The Romanians were caught after a witness called the police. The men were put in custody in the capital awaiting charges.
I wouldn't be to worried about it happening often. On the other hand, add this to your alert horizon. Ask for complete ID, don't hand anyone your wallet, and you should be safe.