How can people tell that I've seen their facebook message?!?

[Ollie]

I had to tabs on Firefox, one with facebook and the other youtube. I was on the youtube tab and some sent me a chat message, so i went onto the facebook tab, but i didn't click on the chat box which was flashing letting me know its a new message and i also didn't click on the chat notification. so without clicking on either of those two things apart from just going back onto the facebook tab would that person get a "seem at" message on their chat box saying i've seen it?

[Ryu]

No, you have to click the chat box or the message notification.

Otherwise it will still just say "Sent at (time)".

But I mean, if they see that you're online, just because it doesn't say "Seen at (time)", they can pretty much assume you saw it.