"It is elsewhere."
I think I'm getting better at knowing when to use "c'est" and when to use "il/elle est," but please let me know if I'm correct here. "Il/elle est ailleurs" would suggest that a particular thing is elsewhere, and "c'est ailleurs" would be... somehow more abstract? I'm actually having trouble thinking up a scenario where I think "c'est ailleurs" might be used. "Where is this place?" "It's elsewhere." I don't know...
This URL(http://french.about.com/od/expressions/a/impersonal.htm) suggest the Il est and C'est are interchangable for a sentence such as "elle est ailleurs".
But it also conflicts with this one: http://french.about.com/library/weekly/aa032500.htm (for unmodified adverb).
I had the same question as everyone else here. A quick google search took me to -> http://www.wordreference.com/fren/ailleurs
The example: "C'est ailleurs qu'il faut chercher la cause de leur divorce." "You need to look elsewhere for the cause of their divorce."
So, I suppose both can be used in different senses. "Il est / elle est ailleurs" when talking about a concrete object (Le dossier est ici? Non, il est ailleurs.), and "c'est ailleurs" when talking about talking about something abstract (as in the example above)