It's not just a Mac problem
The problem is also present on the Philips VOIP841 Skypephone. For ages I could not work out why at work my Windoze PC was quietly running Skype, but on my return home my Philips phone logged into the same accound with status set to 'Away' would show multiple sequests like this at the end of the day. (An interesting Skype feature is that you can be logged in at more than one location and receive calls.)
The problem is more widespread than we perhaps think, and it's made me change the way I use Skype. I now log the phone off when I'm not there, a far from ideal solution.