In the days of analogue TV, they almost certainly did. Analogue CRT televisions put out a tremendous amount of spurious RF. The intermediate frequency from the receiver's local oscillator was easily detected from some distance, even with inexpensive amateur equipment. With more sophisticated equipment, it wouldn't be difficult to see exactly what was on the screen using Van Eck phreaking. With modern technology, you could build a working TV detector bicycle using nothing more than an Android tablet, an RTL-SDR stick and a home-made yagi.

I think that the secrecy was about how often TV detection technology was actually used. The BBC research department could have knocked together a working TV detector van in a spare afternoon, but it would have been prohibitively expensive to operate. Why send out two skilled broadcast engineers in a van full of equipment, when an empty van being driven by a debt collector will do the job?