User Account Self-Deleted???

Is it possible for a standard user to delete their own account?  I have been under the impression that only an Admin could delete forum user accounts, but I saw something strange on my forum this evening.  A new user showed up in the list of active users.  I clicked on the user name and saw that they were browsing one of our forum topics.  Later, I clicked back on the username that was still shown as being active, but I received a yellow error that said something like the user account was not able to be found.  Now, the user is no longer being shown as active, and it's not showing up in the Memberlist.