I recently bought a WiFi repeater to cover a dead area in my house, and it works so well, I am a little concerned. All I had to do, was connect it to my WiFi network with the password, and now it seemlessly repeats my Internet signal. My devices don't seem to know they are communicating via the repeater to the main router at all. It made me wonder, if you knew the guest password to a WiFi anywhere, couldn't you setup a repeater to capture that traffic, and then you'd be able to see all of it? And, if you did, would a VPN be enough to protect people as you saw all the data going to and from their devices? Here's hoping VPNs do the trick. It's a little worrisome.