[mikewarot]

I would assume you could use an SDR dongle to find cell phones (that are on) in the same manner, along with anything using WiFi. (At different frequencies, of course)

Back in the 1980s, we amateur radio operators (I'm KA9DGX) used to call it "Fox Hunting", and did it on a much larger scale, on the 2 meter band.

[MarkusWandel]

One of my kids was (still is to some extent) a "runner". It did occur to me to go low tech, put an oscillator kind of thing on her, and then use one of those helical antenna thingies to DF her. I'm sure schematics for that kind of analog fox hunt stuff would be easy to find. But times being what they are, if you showed up with a rig like that at the playground, it would at minimum be a "conversation starter" and at worst be "call the cops" kind of thing. So instead you have to get the SIM card and security hole equipped kind of thing and do it with your smartphone. Nothing anyone does with a smartphone raises eyebrows...