What Are Sniffing and Geofencing — and Why You Need a Faraday Sleeve in the City

The phone in your pocket only looks idle. Several times a minute, it calls out to nearby networks, listens for other Bluetooth devices and picks up their replies. These brief radio exchanges add up to a picture of your route, your daily routine and the devices around you. All of this information can be collected, analyzed and used to target ads — or even to track you.

What Is Sniffing?

Sniffing is the passive collection of radio packets sent out by a device. No hacking is required: your phone hands this data over on its own. The receiver never replies or transmits anything, so the phone has no way of noticing it. Wi-Fi and Bluetooth are the most common channels for sniffing. Even when your phone isn’t connected to anything, its Wi-Fi chip regularly broadcasts probe requests — short messages asking “Is there a network I know nearby?” — and each one carries the sender’s address. Bluetooth works in much the same way: smartphones, earbuds and smartwatches announce their presence several times a second.

Probe request — a Wi-Fi management frame that a device uses to search for access points. It is broadcast automatically and contains the sender’s MAC address. These are the very frames that shopping malls and trade shows use to count their visitors.

A whole industry has grown up around this. Wi-Fi tracking sensors are available for under $20 a month. A sensor picks up signals within a radius of about 120 meters (roughly 400 feet), logs the addresses of passers-by in a database and exports the resulting audience to ad platforms. Systems like these are installed in shops, cafes, clinics and even airports.

What Is Geofencing?

If sniffing answers the question of who’s nearby, geofencing decides what to do when someone shows up at a particular place. A virtual boundary is drawn around a store, an office or an entire neighborhood, and as soon as a device crosses it, a preset action is triggered. GPS isn’t the only way to pin down your location. Your phone sees the names of nearby Wi-Fi networks, whose coordinates have long been stored in databases. Cell towers narrow your position down to a city block, and inside stores, Bluetooth beacons can place you at a specific shelf.

Advertising ID (IDFA on iOS, AAID on Android) — a unique identifier that ad platforms use to recognize your device. Geofences, interests and purchases are all linked to it. You can reset it in your settings, but your profile can be rebuilt from your device and your behavior.

Sometimes this is genuinely useful: a smart home, for example, can switch on the lights just as you get home. But advertising runs on exactly the same mechanism — except that it isn’t you who draws the boundary. In Google Ads, an advertiser can target a radius as small as 1 kilometer (about 0.6 miles) around any address. Law enforcement uses the same tool. In 2020, Google received more than 11,500 geofence warrants demanding data on every device within a specified area.

Who Profits From This?

Advertisers want location data so they can target a specific neighborhood with precision and see whether someone actually made it to the store. Data brokers make their money differently. Their SDKs are built into weather, flashlight and coupon apps, and your coordinates are sent to their servers even when the app is closed. In 2024, the US Federal Trade Commission banned the data brokers X-Mode Social (now Outlogic) and InMarket from selling sensitive location data. How securely these massive datasets are stored is another story. In January 2025, hackers claimed to have stolen 17 terabytes of location data from the broker Gravy Analytics. The leaked files contained traces of 3,455 apps, along with the coordinates of devices in the US, Europe and Russia.

The Faraday Sleeve: A Solution Grounded in Physics

Privacy settings can reduce the amount of data your phone leaks to third parties, but they can’t shut it off. Permissions get reset after updates, airplane mode can leave Wi-Fi and Bluetooth running, and advertising SDKs play by their own rules. A radio signal isn’t a software setting — it’s a physical phenomenon. And it can only be stopped at the physical level.

A Faraday sleeve blocks electromagnetic radiation, and with it the radio signals your phone relies on. A layer of metallized fabric forms a continuous conductive mesh — in effect, a Faraday cage — that stops radio waves from getting through. That’s why the sleeve cuts off every wireless channel your phone uses: cellular, Wi-Fi, Bluetooth, GPS and NFC. Your phone stays on, but it goes off the air: it stops sending probe requests, doesn’t respond to beacons and becomes invisible to geofencing.

Privacy on Your Terms

Sniffing collects what your phone reveals about itself, and geofencing turns those signals into events on a map. Settings and app permissions can’t solve the privacy problem on their own. A shielding sleeve protects your privacy at the physical level: while your phone is inside, it exchanges no radio signals, so nothing new about you ends up in an ad database. Most importantly, you decide when your phone is connected and when it isn’t.