Facebook message no longer marked as seen?

[Nikki Minaj]

I was just saying goodbye to a facebook friend. What I said was "bye" and it was marked as seen as usual. But then I check the next day and where it should say "seen" like it did yesterday there is nothing, so the message is no longer marked as seen. Is this a glitch or a cache problem or what? And has anybody else had this problem?

[abraXus]

the seen indicator goes away over time

it's also possible that the person marked it as unread after the read it for some reason