Well, a security researcher, Brendan O'Conner, developed a device (which many could make on their own) that can snoop on anyone whoever happens to hook up with unsecured Wi-Fi.
He could pick up the Web sites he browsed when he connected to a public Wi-Fi – say at a cafe – and he scooped up the unique identifier connected to his phone and iPad. Gobs of information traveled over the Internet in the clear, meaning they were entirely unencrypted and simple to scoop up.
Even when he didn’t connect to a Wi-Fi network, his sensors could track his location through Wi-Fi “pings.”Not only that, but it could gather email addresses, Id's to social media sites, photos and a lot more. You can learn more about it by reading A Cheap Spying Tool With a High Creepy Factor
This is not legal to do anywhere, but it can easily be done. So my tip for you is to turn off Wi-Fi except when you are using a secured server. Identity thieves can really have a hey day with something like this.
Whatever you do - Stay Safe!