Posted By Dan D DuBal
If memory serves, Olympus has been imaged via CCD at premier observatories (Pic du Midi, for one, I think) as an extremely faint very-low-contrast "blemish." Images of Olympus-hugging orographic clouds have also been recorded. Not sure if anyone has actually *seen* the feature (or clouds thereupon), but conditions *and* equipment would surely have to be optimum.
As Greg noted, it's Olympus's ultra-low contrast (with surrounding plains) which keeps it hidden.