Why would Facebook say I got a message and nothing appears when I check?

Is it a troll that got reported for harassing strangers or some guy who wanted to know my relationship status?

Updates
+1 y
A fellow contact was hacked. Somebody tricked them into responding and clicking and then used the bait to take over their account. It’s a spambot that was flagged for suspicious activity.
Why would Facebook say I got a message and nothing appears when I check?
Post Opinion