I had the same experience being in downtown SF (near Market) for the first time in a few years, but I attributed it to the number of electric cars.

The whole visit felt weird, and eerie, and off somehow, but I couldn't figure out what it was. And then I was standing waiting for a crossing light and heard the clicking of a scooter's turn signal ~20 feet away. It stood out because it took a few seconds to realize that I shouldn't be hearing it because of other noise.