Hey Chris,
This is not a huge deal, but thought you might find it interesting since you are a fellow Type-A. I checked for unread posts last week on Ken's computer. When I returned home, I checked for unread posts on my own computer, and it came up with the same posts that I had already read from Ken's IP address. I am certain that I was logged into my own account when I was on Ken's computer when I checked for new posts, and specifically remember logging Ken out many times. Additionally, when I checked for unread posts at home today, it came up with old posts that didn't show up last week at Ken's.
I've used different IP addresses before to check for new posts, and have never run into this problem before.
-Trina