Why did my Facebook message go from "seen" to "sent"?

[Emily]

On Facebook, I sent someone an email. For the longest time it said "sent" right at the bottom, so I figured she had not read it yet. Then a couple of days ago, I was online chatting on Facebook through the app on my iPhone, and I checked and the message said "seen at xxxx". So I thought she read my message. Later in the evening it went back to "sent". I have no idea why and I have no idea if she saw my message. Has this happened to anybody else?

[pri✍]

Sent is just as it means the message was sent at xx.xx hrs. "Seen at xx.xx" means the recipient opened that pm at that time.

However, we have the option to Mark Unread the last message, thus delete the "Seen at". It can be found as the first option under Action on top, or on the right of the name in the blue box on the left column. (talking from the view of FB full site)