Anyone can just send a message to any Facebook user, and previously, if the sender was not in the receiver's friends list, the message would land to Messenger's "Other" folder. The Other folder received mixed feelings of reaction. Some felt bad for accidentally ignoring people they want to connect with, and others felt thankful they never see unwanted creepy messages sooner.
That changed in 2015, and messages from non-friends would land to the "Message Requests" instead and the receiver is allowed to decide whether to accept the message or not.
The new feature is Facebook's way to filter messages, otherwise the user's inbox will overflow with junk messages and legitimate items may mess with unwanted ones.
This way of filtering unwanted messages could make some users miss their messages. The notifications are filtered so the user does not see people who are trying to contact them, an attempt to cut down spam messages that may abuse the receiver.
The filter is like a Spam in emails, but it can also be possible that the user miss some legitimate message from people who want to connect, Quartz reported. The Facebook user may have something there that could be useful, especially for those who have their social network account for professional purposes.
Without being aware of these messages, senders may think the receiver just ignored them. For instance, a Facebook user found that in his Message Requests, people asked some questions about things he has written, something that he was willing to respond.
To make sure one has not missed any message in Facebook on their smartphone, they can go to Facebook Messenger app by choosing the gear icon in top right, select People, tap on Message Requests and See Filtered Requests, according to Cosmopolitan.
When using Facebook on a PC or desktop, one can check for hidden messages by opening the icon on the chat box at the top of the page. From there, he can choose Message Requests then click on See Filtered Requests.
The video below talks about hidden messages on Facebook that may have useful ones the receiver does not want to miss.