Your local cafe’s WiFi might reveal who you are, even if you’ve never connected to it. Researchers warn that WiFi networks can identify people without phones or devices, raising major privacy and surveillance concerns for everyday users.

Researchers from the Karlsruhe Institute of Technology (KIT) in Germany have discovered that it’s possible to identify and recognize people solely through WiFi signals, even if they’re not connected to a network or carrying a device at all.

As long as there are WiFi networks nearby, you’re effectively visible. Simply walking by your favorite coffee shop or someone’s home router could leave a digital footprint.

“By observing the propagation of radio waves, we can create an image of the surroundings and the persons there,” said Professor Thorsten Strufe from KIT’s Institute of Information Security and Dependability.

“This works similarly to a normal camera, the difference being that in our case, radio waves instead of light waves are transformed into an image,” explained Strufe. (Read more from “Wait, What? WiFi Signals Can Be Used to ID You Even If You Carry No Device” HERE)