If a facebook message isn't marked as seen, has the person definitely not seen it?

[sarah smiles]

uhm, if you click on messages and go to you actual inbox page thing, for everyone conversation you have, there's the option of "mark as unread."

so, there's always a chance that a person could have read a message and then simply marked it as unread.

[Julianna]

Maybe they just haven't seen it. No matter if your on your phone or on the computer it will say seen so you should contact the person another way maybe

[Ethical Paradox]

Yeah, no the person has to click on the message because it appears as a little red notification on your messages.
I used to wonder the same thing, until I conducted an experiment with my friend.
It could be that the person is actually on another website and hasn't replied to your message, or saw it, but didn't actually click on the notification.
Kind of confusing I know...