For your boyfriend to hide things from you, is it a bad sign?

So, as you all know by now Facebook has this new “nifty” add-on called the chat app. You don’t need AIM, YM, MSN, etc.

Here’s the thing. Today, in the morning, I saw his chat history with this girl on facebook that happened the night before. I understand if they were just talking randomly about finals and so forth.

It gets to the point where she complained how their final (same class, same final exam) was at 8 o’clock in the morning. She started asking him if he can wake her up early to make sure she didnt sleep in. I’m pretty sure, as I am a skeptic, she should be able to wake up early by now, it’s freaking college. So my boyfriend said, sure ill wake you up—6 ok? She said no problem!
It’s not the serious chat, it’s a flirty feeling that I get while I was reading their conversation.

Alright, that’s the chat history. Facebook lets it saved to Message History—where I read the whole thing in the morning. Tonight, I saw, out of all the chat message histories he’s had on that night—whether it was with a guy or a girl, only hers was gone (deleted).

As a girlfriend, should I be alarmed by this?
As a guy, give me a point of view. From my deductions (positive thinking), he doesnt want me to find out about this whole chat history so I won’t misunderstand. My other deduction (negative thinking), finds me at the point where I am putting a red flag on him. What else happened during the evening (while I was gone, adding up to their conversation the night before)?

As a guy, why would you want to delete something if there was nothing to hide? Why do that at all?

Thanks for your input. If any of you have experiences on this matter, please enlighten me. I just dont understand.