If it blew off, you didn't have the bottom secured into the slot/tang. I know of other FJR owners that have traveled some significant distances with unlocked bags, (unintensionally), w/o incident. The latch might have been locked on top, but if you put the bag on correctly, it wouldn't blow off w/o one heck of a bump to get it up and over the latch hooks and the bottom tang.
The number of miles you've ridden doesn't really have anything to do with this. Anyone can accidently put the bag on wrong. Over 9k on the bike huh?