Pretty sloppy handling, imho. If you're going to use an exotic radioactive poison you don't want to leave a bread-crumb-like trail of the stuff the authorities can use to retrace your steps. :/
It could be a false trail intended to incriminate someone else I suppose, but it makes little sense to leave a false trail unless you think someone's going to come looking for it. One advantage of using Polonium-210 in the first place is that it's virtually impossible to detect--so long as no one suspects a radioactive poison. Litvinenko was (mis-)diagnosed and being treated for Thallium poisoning (Thallium is a fairly ordinary and easily obtained poison*, and has similar symptoms even though it kills via chemical reactions and not radiation) right up until about the time he died. They came very close to missing the Polonium-210 altogether and attributing his death to Thallium poisoning or to some unknown cause (and if that had happened, they would not now be looking for and finding traces of Polonium-210).
I would think if you wanted to frame someone you'd use a more conventional means of murder and more conventional types of incriminating evidence (to ensure both that the authorities would look for said evidence and that they would then find it).
*Thallium used to be the active ingredient in many rat and ant poisons (and undoubtably still is in some parts of the world).
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