The Cobalt-60 accident in Juarez back in 1984 is the one I always think about when I hear these things.
What a damn mess that was.
A Mexican guy had a capsule full of it he forced open at some guys house, he had the highly radioactive pellets all over the back of his pickup truck...kids were playing with them, as the truck drove from place to place and they spilled out all over the place.
They took it to a junk yard, the pellets were picked up by one of those big magnets they use to pick up scrap metal, which they sold to a foundry. At the foundry the scrap was melted into a bunch of radioactive table legs, girders and such that found its way into the US.
They had to use helicopters with detection devices, trucks, detectives, etc.
Ugh.
I'm not sure if the cobalt in radiation sources is in the form of the metallic element, or if it is in a compound such as CoCl, which would not be attracted by a magnet.