void on_walk(object ob, int start, int end)

Called when the object ob walks around within a room. The start and end
are the coordinates between which the character is walking. Note that
if they're walking a longer distance, this will probably get called
multiple times. This gets called in the target as well, and is called
even on commands that are not explicitly the 'walk' command (for example,
if you get an object elsewhere in the room, and you have to walk to
it, this event will happen).

Note that this does NOT send you an event when someone leaves the room.
You should look at on_depart for that.
