Similar to friend requests, Facebook Messenger applies a filter to messages received from users who are not yet friended. Here's a quick tip on how to view these pre-filtered messages.
How To View Facebook Messenger Requests
Open the Facebook Messenger
app and tap on the cog
icon > People
> Message Requests
Tap on a message request to view its content > menu
(vertical ellipses) > Delete All
to clear all of the message requests.
Facebook Messenger will not notify the sender that you have viewed the message unless you reply to it.