Threads marked as read even if I haven't read them?

[minameismud]

Here's my normal forum trolling pattern:

B7 A4
Avants Only
MidWest
For Sale (et. al., sometimes)

This morning, I didn't hit the For Sale section at all. I just now went into the FS section and most everying was marked as read, even threads before this morning that I know I haven't read! I do use the "Mark this forum as read" in specific forums when I've read all the new threads I'm interested in, and at the time, it doesn't appear to affect other forums. I've noticed that this happens daily, where if I don't visit the For Sale forum (or MidWest, or Group Buy, etc.) on a visit, the threads will still appear as if I marked them as read on my next visit. Is there a way to turn this off? It's very annoying, especially in the For Sale forum.

[Anthony]

There is a cookie time-out. So for instance if you log in, then you go take a shower or have dinner, when you come back the threads will get marked as read even if you didn't view them or click the "Mark as Read" link.