He gave you the benefit of the doubt and you may well have had a clear conversation with him and he just decided not to.
Some time ago I was leaving a restaurant quite late (it also has a rather large bar). I had been working there with others for a few hours and I didn't drink at all; anyway about 5 mins after leaving this dark car in front of me had no lights so I flashed it .....Now this is just my luck!! the car let me pass it then put sirens on and I was pulled. Police officer came to my window asking me if I had been drinking (he had an unmarked car). (I was very tempted to get a little black book out of the glove compartment!! and start asking him questions) More or less straight away, he backed away from the car and said that it was clear I hadn't been drinking because he couldn't smell it on me and disappeared very quickly - he didn't go any further and I had my wits about me. So he satisfied himself and maybe thats what they do and they must have some discretion.