Facial recognition devices have become ubiquitous across China. But what you probably didn’t know is that some of these machines can snatch up information straight from your smartphone.

While they look like regular metal detectors on the outside, they’re much more than that. Aside from facial recognition and ID card verification, the so-called “three-dimensional portrait and integrated data doors” vacuum up MAC addresses, IMEI numbers and other identifying information from electronic devices. This data is unique to a user’s hardware, and it could potentially be used to track people.

A new report from Human Rights Watch uncovered the use of these data doors at certain checkpoints in Xinjiang, where the government is using heavy surveillance to monitor the local Uyghur Muslim minority.