Indeed. I remember reading a comment, possibly over on ARRSE, that it was easy to out a Walt simply by asking for his service number. Logic being that it's unique to the person and therefore verifiable and it's probably the one thing you will never forget in a hurry, with that specific detail often outlasting the ravages of time. No / incorrect service number is always going to flag up, even amongst the 'Special' types.
There was at least one circumstance in the RAF where an individual would change his service number.
This is when a Boy Entrant transferred to be an Apprentice.
In these cases the B/E's 19xxxxx was replaced by a 58xxxx number.