Here's an interesting observation I recently made when I used a VPN in a hotel and came across a website that asked for my location details in the browser. I was confident Firefox would not be able to find out where I was as I used a VPN tunnel to my gateway in Paris. I thus pressed the 'yes' button, expecting that the website would then tell me that I'm in Paris. Much to my surprise, however, it came up with my exact location. How is that possible, I thought, my IP address points to my VPN server in Paris!?
A detailed answer can be found on Firefox's Geolocation info web page here. In addition to the IP address, Firefox also gets the list of nearby Wi-Fi access points and sends that to Google's location server. At the location there were only two Wi-Fi access points in addition to my own as shown in the screenshot on the left but that's enough for Google to locate me.
Incredible on the one hand and scary on the other. It's no problem in this case as Firefox asked me for permission before sending the data to Google and the web page. But it shows how easy 'others' can pinpoint your location if they manage to get a little piece of software on any connected device you carry that has a Wi-Fi interface.