On the other hand if the assassin(s) knew it was traceable it would be like shooting a flare gun at midnight.
So it depends - did the assassin(s) know Polonium-210 poisoning was traceable? When they catch them they can ask them.
A very expensive poison - unless you stole it somehow. So does that mean a government (probably Russian) involvement or not?
Russia exports Polonium-210 for commercial uses. If someone knows where this batch was made, as far as I know they haven't leaked it to the press.
One thing that does seem to indicate Russian involvement - they seem very uninterested in cooperating with the investigation. You'd think the possibility of Polonium-210 winding up in his own tea would concern Putin.