Once you close(resolve, move to Done) an issue directly from the backlog list (not sprint list), using the Details Menu-> More Actions -> Done, the issue will disappear and won't be accessible from the board or backlog. If you go to the issue manually (via issue search screen) and click View on Board you'll get "There are no boards which contain this issue." message.
Without going to the Issue Search screen we're not able to tell if there are issues closed like that... we may miss some tasks that way....
Any ideas if this is a bug or just expected result?
Cheers
Prem