I've found that they'll only move if stressed, overcrowded (which kind of goes to "stressed" I suppose), or in some cases, just about to split.
I suppose if you buy one from the store, it might still be adjusting to who-knows-what-it's-been-though, and likely light changes, parameter changes, so they might wander a bit until used to their new surroundings.
But every captive split I've moved myself, I'd create a spot that I'd "think" would be to its liking, and usually they would stay very well put.
With a BTA you want a hard smooth surface for it to attach to, but it should be an underhang, crevasse, or cave (or something like that); because (I've noticed) they seem to usually like a spot where they can keep their tentacles out in the open (light and current) but their pedal disk (foot) sheltered from both.
Basic answer is, they shouldn't be moving about, if "everything" is to their liking. Wandering behaviour means "something's up."
HTH!
