We occasionally get something like this reported to us against Chrome (people assume the browser is doing something fishy). So far it's always turned out that someone else simply logged in to check their email without the primary computer owner's knowledge. So, the first thing I'd suggest checking is if anyone else may have used the computer, or perhaps let a visiting friend use it. It's the holidays after all, and most people have a lot more traffic around the house than usual.
This isn't to say there couldn't be a bug in the session handling, just that my repeated experience has shown the human element to be far more likely.