Session expiration

I recently moved to memcache for session management and I am very
happy with the performance.

However, I need a way to trigger methods when a user is logged out,
including when the session timer expires. I can think of a few ways
to do this when using file and ActiveRecord sessions, but is there a
way to do this with memcache, or better yet a method that is not
dependent on the session store.

Any ideas?