I have a Gmail account with over 50K messages (my rss feeds) and I sometimes read it via Gmail mobile. So, I end up with messages that have been archived but not labeled (oh no!)
To find them, search this:
"-label:inbox -label:blog -label:comp ..."
You have to add a -label:tag for each tag, so it can take a while to write. But bookmark it and you never have to write it again! I found the idea at the bottom of this thread.