Here's something of interest.
It is only the player's party that can cross over treasure boxes. Guards, townsfolk, merchants, and, yes, monsters cannot. You can use this knowledge to create walls of treasure boxes in the overworld (where they don't disappear until you open them - unlike on sub-maps which reset when you leave) to make safe spots or to wall off important areas. It takes some monster movement manipulation, but it can be very worthwhile.
This also comes in handy on those rare occasions where you want to not just steal from a merchant, but really rob him blind. If you have some way to get behind the counter, and you leave a wall of treasure chests between you and him, he can't reach you to start a fight. Merchants move and operate like monsters - always homing in on the party's exact location, and starting a fight whenever the party closes to range - so normally crossing behind the counter is dangerous, since the guards don't really care who starts the fight. But if you wait until after getting back there to take out the row of boxes that you can reach with the Steal command, you can take him for every last cent. Then leave the towne sub-map, lather, rinse and repeat.

Note that this applies only to Ultima III.