Cant help with a replacement, but to relate a similar story when mine flew off on the way down to Sydney. Happened at the worst possible spot where there is no chance of recovery at any time of day or night so I had to leave it. Found out after I received a new one that there is supposed to be a cable tie attached to the cover and screwed to the frame which helps to prevent then flying off. My bike, which I've had from new, was not supplied with these hence the problem.
Might be worth checking your other mirror before it too goes walkabout.
