There was another theory (spread by the media and by the half-denying-half-encouraging bbc) that the vans can detect iPlayer usage across WiFi by looking at the level of network traffic, and then to a range of IP addresses in an area intentionally changing the amount of data the app received up/down to see if network activity changed correspondingly.

https://www.express.co.uk/life-style/science-technology/6976...

It’s really all a big scare tactic though. The best way to detect iPlayer being used wrongly will be to see who registers for an account with an identifiable email address.