This was initially posted by me in the mtgUK Rules & Judges Questions Facebook group:

Player A controls a Merieke Ri Berit and gains control of Player B's Runeclaw Bear by user her tap ability. Player C casts Control Magic on Merieke so the Bear returns to Player B's control without triggering the destroy clause.
The question is now: if at any point Merieke becomes untapped, does her destroy trigger happen or did the ability cease to track the Bear when it returned to its original owner's control?

This was discussed to some extend with no real definite conclusion.

Parts of the comprehensive rules that were searched for the answer include the following:

603.7c A delayed triggered ability that refers to a particular object still affects it even if the object changes characteristics. However, if that object is no longer in the zone it’s expected to be in at the time the delayed triggered ability resolves, the ability won’t affect it.

603.10. Some objects have a static ability that’s linked to one or more triggered abilities. (See rule 607, “Linked Abilities.”) These objects combine the abilities into one paragraph, with the static ability first, followed by each triggered ability that’s linked to it. A very few objects have triggered abilities which are written with the trigger condition in the middle of the ability, rather than at the beginning.

607.2g If an object has both a static ability and one or more triggered abilities printed on it in the same paragraph, each of those triggered abilities is linked to the static ability. Each triggered ability refers only to actions taken as a result of the static ability. See rule 603.10.

611.2a A continuous effect generated by the resolution of a spell or ability lasts as long as stated by the spell or ability creating it (such as “until end of turn”).

Edited Achim Unland (July 7, 2015 06:44:01 PM)

Yes, the Bear will be destroyed if Merieke untaps or leaves the battlefield.

Merieke's ability does two things as it resolves: it sets up a continuous effect that changes control of Runeclaw Bear for as long as Player A controls Merieke, and it also sets up a delayed trigger that will destroy the Bear the next time Merieke untaps or leaves the battlefield. These two things function independently of each other–the triggered ability doesn't care whether or not the control-change is still in effect when it triggers.

(603.10 and 607.2g are not relevant here, since Merieke's tap ability is activated, not static.)