Strange Behavior

The other day, as usual, I was slugging home. Unusually, however, the driver was obsessed...with license plates. She had a piece of paper in her lap, and whenever a car would go by, or she'd pass a car that had 4 numbers on the plate, she'd pick up the paper and scribble it on the end of the list. She had about 20 numbers printed on one side of the paper, and sometimes instead of writing the number, she'd cross it off. Strange behavior, to be sure, but there might have been an explanation for it. I didn't want to ask, just in case there was. I prefer to believe that people are strange.