If it's like the Givi one I've had for years on various machines, you're right. It sounds very much like you didn't put it on. There's no way to have accidentally put such a box on insecurely, and you could hardly be blamed for the box falling off due to a failure of the securing mechanism, so any suggestion that you should pack in riding is, frankly, risible.
I remember Julian's box falling off, I think it was a toss up as to whether it was Mike or I who got on the radio first! I can't remember how many times it bounced before it came to rest and, much to its credit, it didn't burst. That was a good thing, because it contained Julian's underpants from the tour we'd just been on
I can well believe someone has picked it up - people will nick stuff first and ask questions later. Whether the thing they've stolen is any use to them or not is a different matter...