I thought I'd been rooted when I saw a notification pop up on my desktop with no previous actions on my part -- it just appeared out of nowhere for no apparent reason, offering to sell me gold dust (cheap!).
Turns out that evolution (a gnome clone of MS Outook) was checking my google calendar account automatically and I didn't even know it. I use gmail every day, but never use the google calendar app that comes bundled with the gmail account. I do use the evolution calendar function on this local machine for popup reminders of birthdays, etc, but never do I publish my calendar on the internet. So, be aware that some calendar apps are set to do this insane stuff by default -- and google calendar accepts unsolicited invitations by default(!). Maybe you should turn that off if you're not expecting a Nigerian offer you can't refuse?
