When you look at the details of a request in the Messages queue, the authorization status changes automatically to "Acknowledged". This causes the request to be automatically removed from the queue unless you select "mark as unread" at the top of the screen. "Mark as unread" changes the authorization status back to "To be acknowledged" and keeps the request in the Messages queue.
If a request has already disappeared from the Messages queue, it can be retrieved using the Advanced search by selecting Last Action = Send Note-indication. This search calls up all requests with new messages, even if they have already been read.