I've always been good at making guesses about who people are, and often used the skill when I was recruiting informants. I generally kept what I knew to myself though; my talent for observation put most people on guard, and my memory could cause the same problem. When it's a question of how much you remember about an acquaintance, a colleague, or a snitch, there's a fine line between what's flattering and what gives them the creeps.
The officer gave me some deliberately direct eye contact, his face wide open, his eyebrows up as he smiled slightly. When I first approached a target, I did so with this very same look on my face. It was a basic step, one of many I used to suggest to my targets that I was trustworthy. I imitated his expression; right from the start we were working each other.