Rule 383.2.c.3.b
When the Triggered Ability is being processed, note any information about the source of the Triggered Ability or the state of the game as needed in order to process the resulting Triggered Ability. Example: A unit reads in part “When I leave the board, deal 2 to a unit where I was.” Its controller plays Retreat on the unit, returning it to hand. Before the unit is returned to hand, the triggered ability is placed on the chain and the necessary information about that unit is noted. When the triggered ability is being finalized, we can identify the former location of the unit in question with the noted information.