It actually happens when I click the "show unread posts since last visit" link although it is a bit more subtle than hitting F5. The noticeable part is the banner and top "frame" with the various links, buttons, etc. I don't notice when clicking on other links.
Looking at your UA, it looks like you're using IE8. I don't have a machine with IE8 nearby to test with but it sounds like something IE8 would do. Possibly not honoring the expires headers. I'm not sure, but I'll look when I get a chance, and in the meantime revise my answer to "probably still no."
I haven't looked at the source so forgive me if that area of the page some new fancy pants version of a frame.
I'm not sure if this was sarcasm or not, but I'm just telling you what I know
And no it's not a frame or anything like it, just standard table/tr/td and div tags with some style sheets.
PS - you can use the DC logo at the right end of the banner for recent unread. Not sure if you were being literal when you said "'show unread posts since last visit' link"