If you have used Facebook's Messenger app, you have probably seen the "Other" inbox -- but did you know there's yet another place where messages might be hidden?
Facebook has a feature that sends you a "message request" when someone you are not friends with tries to send you a message. Those messages are being filtered.
To view the "filtered" messages:
Open Messenger app > Settings > People > Message Requests > See Filtered requests
The majority of the messages will probably be spam, but you may have some missed messages from old high school friends trying to get in touch with you.