> they want to filter out people who don't take OpSec rules seriously

I personally think it's a little bit this.

I know someone who applied to be an intelligence analyst agent in the UK. First stage was a multiple choice exam in a London hotel, where they had to turn up and say they were there for "KRNP Recruiting" or something. Now the hotel staff must have known something was going on, and it wouldn't be to hard to find this out ... but I reckon it filtered out both ppl who didn't take it seriously and also people who enjoyed that kind of subterfuge a little to much :-)

(honestly can't remember exact letters now)

That was my understanding, they know it is possible for a foreign government to tap the phone lines and they don't want a foreign government being able to target candidates for hire.