facebook seen time gone?

[Katie]

hey guys.
i just noticed something peculiar i've never really seen before.
i sent someone a message, and got that annoying/sometimes-helpful "seen" timestamp like usual whenever they saw it
then a few hours later when i was looking at the conversation, the "seen" time is completely gone.
neither me nor the person has sent another message since that timestemp. do you have any idea what could cause that time to disappear?
I think I just found the answer to my own question. The timestamp can also disappear if you delete the conversation. Not just an individual message, but the entire conversation.

[abraXus]

maybe the person marked it as "unread" or maybe the seen time was already seen by you so facebook doesnt think you need to see it anymore