I once knew someone whose outboard leaped off the back of his boat (not a SIB, but the principle is the same) so it can happen.
I think if you attach it carefully in the first place and check it periodically, the risk should be negligible.
I have just bought one of those outboard locks, mainly to comply with the insurance.
What you do is line up the thumbscrews (tightening levers) then slot the lock over them. Therefore it is impossible for the bolts to turn - and therefore the engine cannot be removed either deliberately or accidentally.
Of course, if someone really wanted to swipe the engine, 30 seconds with a jigsaw would cut a bit out of the transom, including the mounting plate, and they could worry about the outboard lock later. However the lock will prevent accidental loss of the engine and may deter opportunists.