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?
Ask to see their warrant or tell them to get lost. They have no power of entry, possibly unless accompanied by the po po (in which case ask the same question and "Am I legally obliged to comply with this officer?", while videoing them and their response).
> Enquiry officers do not have any legal powers to enter your home without a search warrant granted by a magistrate (or sheriff in Scotland). > You have no obligation to grant entry to an enquiry officer if you don’t wish to do so.