Was locking up my bike this morning, in the racks in front of my building, (not secure storage) and noticed that mountain-bike next to mine had very flimsy lock which was attached to Sheffield rack, but just looped over saddle stem. So, you could ride off without undoing the lock, but would leave lock behind. If you actually wanted to take lock with you, you'd have to get out your key and unlock it.
Was so baffled that I stopped to take a couple of pictures on my phone (would post them but I don't know how to get them off phone).
Then, at lunch, wandered past the bike, noticed bike (and lock) were gone. THEN, looked at my bike and saw that I'd never actually locked my lock on. Boy did I feel stupid. (To be fair, the lock is a bit tricky, I don't think I'd completely neglected to lock it, just that I'd not checked that it had 'latched')
But, that'll teach me to laugh at other people's locks!