FWIW you can do much the same thing with your own SDR setup. One of the more surprising things for me was that the feature that a phone work "internationally" m

FWIW you can do much the same thing with your own SDR setup. One of the more surprising things for me was that the feature that a phone work "internationally" means that a nominally "4G" phones will still answer a GSM tower (talking on a GSM frequency) when the tower says hello. Some phones will let you turn that off. But that said, most smartphones will tell you their WiFi MAC address if you tell them you are an access point. It is more difficult to track a MAC address back to its owner, but it is easy to see if it shows up again near you. My Cisco access point did a variant on this when MAC address filtering was on, it would send you reports of "unknown" MAC addresses which you could log and then later associate with people visiting the office. Bottom line though seems to be to treat protests like DefCon events if you don't want to leak PII. Get a burner phone for such trips.