I have an ASP.Net 4.0 website running in IIS 7 on Windows Server 2008 R2

Having more then one question won't yield you any better results - it'll split the attention of the audience and you'll get one-vote for duplicating.

Check if you're clearing sessions on app exit. Also, abandoned sessions will be cleared after some time unless they are keeping open some active connections. So, clear behind your code and everything should be fine.