Short answer: No where.
It’s still there.
In the Season 4 finale, Ben Linus is explaining time travel [specifically "time travelling bunnies" - ha!] to John Locke in the Orchid station. Somewhere in the midst of that converstaion, he notes that if an object goes forward in time it vanishes in the present. It disappears. When you catch up to it in the future, it reappears.
Ben moved the entire Island forward in time. Not just the people on it – the entire Island!
Let’s say, he moved it forward one year.
It moves. It vanishes from the present. For all intents and purposes, it ceases to exist until…
One year later. It re-appears.
I believe the Island has been moved forward in time before. I believe that the Black Rock was sailing along on open ocean when suddenly it found itself on an Island that had caught up with its future appointment in time.
A lot of people have been thrown off by the fact that a polar bear and Ben Linus ended up in a desert, and so moved both in time AND space. Those are smaller objects. I think the energy requirements to move the Island through both space and time would be prohibitive. I think it’s where it’s always been, just not when.
Others have felt the need to account for spacial movement as well as temporal movement, partially to account for the polar bear and Linus and partly because they feel the need to explain why no one can find the Island.
I submit that we can already account for why no one can find the Island:
- Periodically, due to being moved forward through time, the Island is simply NOT THERE!
- An electromagnetic anamoly throws off all tracking instrumentation, making the Island very difficult to find when it IS there.
- Anyone who gets close enough to the Island finds themselves victim to the Casimir effect, likely caused by the electromagnetic anamoly which is in turn likely caused by pockets of exotic matter indigenous to the Island. The Casimir effect causes paradoxical mental temporal displacement which occurs at increasing frequency until the victim suffers from distraction, dementia and finally death. Unless he or she can find an anchor. This makes the likelihood of anyone casually stumbling upon the Island pretty much nil, since most folks would rationally sail away from this dangerous deterent.