Hmmmm, interesting.... I know exactly what you are refering to, as I lock items in my bags like that all the time!
I've just looked through my bindings to see if I could spot where my alt+button0 was, but nowhere to be found. I can see options in there for ctrl+button0 for dressing room, and similar for sending an item link to chat, and also the ctrl+t binding is there too for the lock.
It seems the binding has been overlooked and should be listed, but unfortunately is not. Which does leave you only one way to re-assign it - and thats to reset everything
All posts to be taken with a pinch of tasty salt.... preferably rubbed on a Tater