Nice semantics
From your statements you are suggesting that:
a. One knows at one point the mountain was physically present.
b. The mountain is no longer physically present (as a mountain).
c. The observer did not see the transition from there being a mountain present to the lack of mountain being present.
So, one really couldn't know if it was destroyed, went out of existance, beamed onto a UFO, made invisible by some secret government project, etc. One can only say that it is no longer part of the present sky-line.
