Alpha radiation has very little in the way of penetration. I can be basically shielded by tin foil.
As the Pu238 decays it will transform into other materials that will probably emit beta and gamma radiation which could be detected. And also the shielding might get compromised over time (hopefully not!)
But at least when it was new this would not have been an option. Heat was the only detectable emission coming off it.
But my point was, even the fuel tubes themselves will have more than adequate shielding against the alpha particles emitted. So it's not even that dangerous that the Sherpas were using them to heat their beds :)
By the way I have since looked it up and Plutonium 238 decays to Uranium 234 which is also an alpha emitter but with a huge half-life (235.000 years). So it looks lke it does not actually produce more penetrating radiation as it decays. The uranium will degrade to an unstable thorium isotope eventually too but this will be quite minimal due to the long half-life.
As the Pu238 decays it will transform into other materials that will probably emit beta and gamma radiation which could be detected. And also the shielding might get compromised over time (hopefully not!)
But at least when it was new this would not have been an option. Heat was the only detectable emission coming off it.