You can tell which direction a radio signal is coming from, using a directional antenna like a yagi, you literally sweep the antenna around and listen for where the signal is strongest.

This is valid, but old school.

Modern WiFi routers use a technology called MIMO. Basically, they use multiple antennas. Each one picks up the same incoming signal at a slightly different time. Once the router knows the time difference, they can listen only for signals with that specific difference. This allows them to focus on signals from thst specific directiin, and reject noise from different sources.

The router can then turn around and send outgoing signals at slightly different timings on each antenna. This strengthens the signal in the direction of the intended receiver, but not in other directions.

The underlying technology is certainly capable of direction finding, but it does so instantaneously, for every signal it hears.

This technology is broadly available in consumer-grade electronics; it is certainly available to the “radio cops” of the FCC, if they choose to use it.