It is a metal, in the same group as magnesium, zinc and cadmium. It just happens to be in a molten state and evaporates at normal room temperatures. You can see the highly toxic vapor with UV light. It certainly amalgamates with aircraft alloys.
Where did this stuff come from? It is sealed in some lamp units, tilt switches etc as well as the more visible use in thermometers and barometers.
Was it on a HazMat list? If not, why not.