there is actually a few ways the website could be doing it.A while back I tried setting up a vpn for anonmyity's sake. I would go to the whats my Ip style websites and everything was good. then I noticed on some sites and with certain web apps the site would be able to find my location.
I did some digging and found out a lot of them are using googles database which gathers info from the router you are on. you can "_nomap" at the end of your routers SSID however (although im not sure if you have access to the routers while traveling) http://searchengineland.com/google-announces-nomap-wifi-optout-101134.
for me that did the trick and have not run into problems since.