DMG 271 describes the disarm action and states that if the attack roll is successful, the defender "drops the item."
SRD 92 lists things you can do on your turn in tandem with your move and action. One of these options is "pick up a dropped axe," which I can infer means "pick up a dropped anything."
So by the rules as written, can you disarm someone as an action, and then just take the item at no additional cost? Is this really how stealing happens in combat? (I'd always kind of imagined that Sleight of Hand would be involved somehow)
(If it doesn't, what does?)