Rule No. 3: They all seemed to be perfectly embedded into the pavement. Some were smudged with asphalt.

There were a few odd finds that challenged the rules: a domino embedded in a gravelly footpath in Forest Park; two casino tokens in another stretch of pavement; and a possibly-connected phenomenon of nine sets of golf balls embedded into city streets in seemingly thought-out formations such as lines and triangles.

“With this kind of upheaval of general conventions, what will be next, a domino sewn into the carpet in the Arch viewing deck?” Grothoff wrote.

Eventually Grothoff landed on a theory: “I think it has to be a city worker signing his work,” he said last week. “Maybe it’s a creative outlet for a guy on one of the street crews. Who else could be there every time a road is paved in the city?”

Grothoff stopped finding new dominoes around 2014. He still spots many of the old game tiles on his bike rides, though some have been paved over or dislodged.

Grothoff never asked the street department about the mystery, though he did once spot a paving crew while he was on a ride. He thought about asking about the dominoes, but they were working, and at the time he thought knowing the answer might have ruined the hunt.