Too many false positives: recorded videos, video games, computers

There's a bit too much confusion as to what era the BBC vans might have been using optical means instead of RF means.

In the 1950s there wasn't much else on British television sets other than the BBC, right?

It doesn’t matter, the offence is “watch TV without a license” (and previously was, I think, “possess a device capable of watching TV”), not specifically the BBC.

(iPlayer is not important to this discussion but complicates things a bit).

Presumably you could compare it to a reference signal, i.e. what is being broadcast right at that moment? Hence the TV aerials on the roof of the vans?

Ever seen the flickering light of a television behind someone's curtains?

Ever seen an app that can identify what song is playing?