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Pure conjecture:
It would be interesting to know if the poor fellow was a smoker. Po210 could have been placed in a cigarette, or pack, or a cigar as it readily vaporizes. The latter would provide excuse for a box that would provide adequate shielding for the assassin, although people don't inhale cigars.
Perhaps a fancy cigarette holder, which would narrow the list of suspects as such items are relatively rare nowadays.
Another reason to kick the habit.