Pardon my French ---- but DAMN that's a lot of radioactivity. (And I work with the stuff for a living.... radiation (outside of a nuclear bomb itself) radioactivity is extremely energetic at the nucleus level of the atom .... but almost neglible at the physical (real world)level. If these things were killing people after only a few hours exposure, and were melting snow ..... they were extremely dangerous.
So the REAL question becomes: What were they built for? How did they get "lost" from original accounting? (This would be like NASA or the CIA/NSA "losing" one of their spare telescopes for the Hubble satellite.
WHY was it laying there to be picked up in the middle of nowhere? Who left it laying there? Where did they come from?
Most important: WHAT GOT THE SOURCES THERE in the first place? (Who else died carrying them?)
An isotope like Co60 puts out heat, I think it is 17 watts per gram. You put the isotope in a superinsulating blanket and use the heat to make electricity, perhaps with a peltier stack. Cobalt and Strontium were common but without lots of shielding would be pretty hot- a gram of cobalt 60 is 1,000 curies, or about 10 to the thirteenth disintegratins per second.
When less shielding was called for, the RTGs may use plutonium 238 (not the Pu239 used in fission bombs) because it is an alpha emitter and does not generate lots of gamma which is hard to shield.
A few years ago the Christic Institute got all upset because NASA was going to launch a bird with lots of Pu238 as an electrical power source, and the fear was that if it fell to earth it would, of course, kill us all. Apollo used one too, see the link.
If someone stole a cobalt or strontium or iridium RTG, and removed the isotope package from its heavy shielding, it may emit on the order of several hundered rems per hour, enough to kill in a few hours. It would be quite warm.
As a terrorist weapon, not so potent except for the fear.
A plutonium RTG like the one used on Cassini, with 11 kg of fuel, could not explode itself but if ground up? It would a great mess make.