Yes, this has happened many times with medical radioisotopes. People find them in a scrap yard, get amazed by the glowing blue powder, and leave a deadly trail of contamination until it's discovered what happened.
No, plutonium is nowhere near as dangerous as the medical stuff. The capsules out of a therapy unit can easily kill, Pu-238 and Pu-239 can't hurt you unless they get inside your body or unless you're cooked by the Pu-238.