[ginge @28
Yes, lead and potassium would be an alloy - a mixture rather than a compound. I don't think anyone would try to make it - the reactivity of potassium would make it unusable - but I suspect that it would be one of the "unmakeable alloys", as the contrast in densities would result in the metals separating before solidifying.
Conclusive evidence of Earth having been visited by space aliens would be to find an artefact made of, for example, aluminium/lead alloy, as this could only be made in zero-gravity (free-fall) conditions. Some experiments on the space station have been performed to prepare alloys of this sort.]