I have a hypothesis that the polonium poisoners weren't planning on being detected.

Polonium has no side effects other than radiation poisoning which can't be detected outside the body. To diagnose it, one would have to go looking for trace amounts in the body or in contaminated areas. If the poisoners weren't aware of the extreme sensitivity of alpha-particle detection -- reliably accurate to the single particle level -- they might have thought polonium would be undetectable and untraceable. Or they might of thought nobody would ever think to look for it. By some accounts, they almost didn't.