I don't know what the S A S's problem is because there are thousands of them. I know this because every other person I meet in a pub has whispered.
"I shouldn't really tell you this, but I used to be in the S A S."
Owning a shop selling militaria for a number of years I used to get "nose tappers" like this on a regular basis. A few well chosen questions would have revealed the truth very quickly, but I just used to let them ramble on, they were harmless, after all.