In Brazil, in the 1980s, a couple of burglars broke into an abandoned hospital and stole a supply of cesium chloride from a radiotherapy machine. The difference between your scenario and this one is that the thieves didn't know what it was -- it just looked important and expensive. The whole thing was a huge screw-up, and a number of people -- including one of the burglars -- died from exposure to radiation before the authorities sorted it all out.

    https://en.wikipedia.org/wiki/Goi%C3%A2nia_accident