Last summer was the first time I noticed them. I was walking to get my lunch when I saw one of them. I thought it was a one time deal until I saw another a few days later as I was heading in a totally different direction.
Since then it's become sort of a game to me. I try to spot the little "aliens" imprinted on crosswalks in the Loop area. It kind of reminds me of Sex and the City when Jack Berger would pick up playing cards.
Not all of the "aliens" I've spotted have been yellow . . .
I will say one thing about all this... Taking pictures while walking through a crosswalk is difficult sometimes. There have been times when there is no traffic so I can stand still and take the pictures; however, most of these pictures were taken during heavy traffic times so I had to learn how to get the shot and snap the picture fast -- without getting hit by a car.