Your polonium was probably actually “extremely low grade polonium-bearing ore” which is to say “uranium-bearing ore.” If there’s still a smidge of rock in there, it’s probably still (trivially) radioactive from uranium, and the reason you can’t see it is that the material emitting the flash has degraded.
Radium resellers back in the day made up business cards with self-luminescent paint. People stuck them in drawers for the novelty, so they’re still around. But they don’t work. Because the radium was fake? Nope, because years of constant irradiation broke down the fluorescing pigments into non-fluorescing compounds. (And also usually because the radium also degraded the binding glue, so the whole spot flaked away. But still, don’t, like, lick one.)
