Curiosity made me dig into this one, because I’ve had friends freak out when a message vanished mid-conversation. The blunt truth is that when someone uses the 'unsend' (or 'remove for everyone') feature on Messenger, the message is typically deleted from both sides and there's no official 'undelete' button. Still, there are a few legitimate, non-invasive routes I’ve tried or seen work that are worth checking before you resign yourself to a mystery.
First, check your notifications. On Android phones you can turn on Notification history (Settings > Notifications > Notification history on many devices) and review recent message previews; some devices like Samsung keep a notification log under Notifications > Advanced settings > Notification history. If you caught a preview on your lock screen or in the notification shade, that text can linger even if the message was removed. On iPhone there isn’t a built-in notification archive, so if you didn’t see it there, it’s gone from that angle.
Next, look into linked places: email notifications, other recipients, or screenshots. If you have Messenger notifications emailed to you, the snippet might be in your inbox. If the message was sent in a group, someone else might still have it. You can also request a copy of your Facebook data (Settings > Your Facebook Information > Download Your Information) and select Messages — it’s a long shot because unsent messages may not be included, but it’s harmless to try.
Finally, I’ll say this plainly: avoid shady apps promising to recover deleted messages by harvesting credentials — that’s a privacy and security risk. If it matters emotionally or legally, sometimes the simplest route is to ask the sender directly. I’ve had awkward conversations end up more human than sneaky recovery ever would, and that’s a little comforting to me.