My understanding is that polonium is so hot that the salad bowl he used was easily found with radiation detectors weeks later, even after being washed in the restaurant’s kitchen dishwasher many times. It was so radioactive from the brief sprinkling of polonium, that it contaminated other dishes that were stored next to it for any time.
P-210 is nasty stuff. Its half life is about 6 months. It would linger for a few years at least.