I agree with jovery.

At my last job we used Terminal Servers and had this problem all the time. Users would CTRL-ALT-DEL their session, and it would never go away. We would kill them, but they still stuck around. Like jovery said, the os doesn't realize it's gone so pmon won't pick it up. Dead-connection detection won't work because it looks for clients *machines* that it can't reach.. and the Terminal Server is still there usually.

The way we got rid of them is to bounce the DB. Until we got time to bounce the DB, we just ignored them.

Good Luck!
Jodie