Over at http://www.hmv.co.uk, their tracklist for Meteora claims that Session is an instrumental, so it's very possible this is Mr. Hahn's solo track. Thanks to LinkinLuke for the news.
yea..i know it'll be sumtin hanh'll do..he's so far alwayz had that spot on tha cd's for a "session" to rip tha vy-nil up.