"Ridin' my Mercury, up and down that road..."
I would think that a car could be contaminated by one of the organic forms of Hg, and that some of it could have been "reduced" (if that is the right term) to form visible metallic mercury by exposure to something (atmospheric pollution maybe?). Elemental Hg is not easily absorbed by the body although just as poisonous as monomethelmercury.
Another KGB hit using poison.
Waiting for Putin’s statement...”c’mon! It’s a setup! I was framed!”