It looks to me like this is one of those things that stretches into the realm of "DM's Discretion".
As with most other effects applied by powers, the dominated status is always accompanied by specific details as to what ends the condition. Typically, this is in the form of "save ends", "end of your next turn", or "end of the encounter", but occasionally other situations may be called out.
Aside from that, I have not found any restriction in the general rules for the dominated condition (pages 230-231 of the Essentials Rules Compendium) that allow for premature termination. Even if the dominator tries to force the subjected creature to cross into hindering terrain, the creature only gets a save against entering the terrain - not the domination.
For loss of line of sight or line of effect, this leaves the field of rulings wide open. One could say that per RAW, this does not end the condition. However, general logic could be argued based on the function (flavor text, not game mechanics) of the power in use to say that it should or should not be ended in such scenarios. It could also be argued that, depending on the function, a dominator may or may not be able to use the subject creature to attack squares or creatures that the dominator does not have line of sight to, or that such attacks would suffer roll penalties.
As for the dominator leaving the encounter, this would have to raise some additional questions:
Firstly, has the encounter ended? Any powers' effects that last until the end of the encounter (typically, the longest duration for a condition) are either terminated at the encounter's conclusion or after five minutes, whichever is first. If the encounter in whole has not ended, is it considered ended for the dominator and should effects applied by that creature therefore be ended? One could logically consider a character's premature departure from a live encounter as being "removed from play". This condition (page 233, Essentials Rules Compendium) still includes the creature in the encounter, while still keeping them far separated from it.
Secondly, does the dominator require the ability to take actions in the encounter? If we're logically applying "removed from play" to the dominator, the rules for that condition specify that the subject creature cannot take actions. However, the rules for the "dominated" effect state "...the dominator chooses a single action for the creature to take on the creatures turn...". This seems to indicate that the dominator does not require any ability to act, as all actions are at the dominated creature's expense and are taken on the dominated creature's turn.
Third, one may also question whether a dominator requires a turn in the initiative order. The first argument against this can be found in the "dominated" condition's rules as mentioned above - all actions are on the subject creature's turn, not the dominator's. The second argument against this would apply if it is agreed that the dominator is subjected to the "removed from play" condition. In such case, the rule specifies that the removed "...creature's turns start and end as normal" thus making this a moot point.
Fourth and final for this post, one would have to question whether line of sight and/or line of effect are maintained, and then perhaps return to the question of whether they are required. This could go a couple different ways. Depending on the landscape of the battlefield and its surrounding areas, it could be physically possible that a departed dominator can indeed maintain line of sight and effect while at the same time being out of range of the general combat. However, if it is ruled that the dominator is indeed "removed from play", then they by RAW have neither line of sight nor line of effect to anything.
Again, as always, the DM is the final arbiter at the game table. So, in cases like these where the rules are fairly ambiguous (and technically, even in cases where they are very explicit) it's their call as to how those rules will be interpreted and applied - or not - in the game at hand.