A puzzling event

A cop car followed me home a few nights ago and pulled up behind me when I pulled up.  I hadn't been speeding and the cop did not allege that I had.  But he asked me to do a breath test.  I guess there must have been something about my driving that looked over-confident or something.

Anyway, I had drunk only a small amount of wine with my dinner so would have been within the allowable limit.  Just to be on the safe side, however, I applied my special technique and registered 0.00 on his alcometer.  He looked surprised and I was amused  but that was the end of it.