Pretty sure this was outted as fake. The idea of a van watching you is enough for many to change their behavior
I believe the current theory is they simply look for light leakage. If they see a "flash - 200ms - flash - 400ms - flash - 350ms - flash" pattern leaking through your curtains precisely at the moment the same pattern occurs in a live broadcast they know you're watching.
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.