I got a notification claiming I was logged in "Somewhere else". I think that's impossible due that I keep protected my account with Steam-Guard, plus I don't use Windows. Can this be a false positive?

We have implement an IP check for detecting multiple IPs logging in with the same browser session. There could be false alarm if your home IP is not static, or you have lost connection to the site abruptly. But you can just simply log out and log in again.