3 Works in Security Clearance
Prompt fill: John's security clearance
John has a high security clearance because of his time in the Army, and that fact is revealed during a case.
Bonus points if it's a surprise for everyone: Yarders and Sherlock.
Sherlock frequently worked with highly classified material in his home; any potential flat-mate would need to be vetted.
Women beware women, in days of auld lang syne.