I noticed those when they first started appearing, my first thought was "must be Street View", followed immediately by wondering about the potential for poisoni

I noticed those when they first started appearing, my first thought was "must be Street View", followed immediately by wondering about the potential for poisoning some kind of machine learning effort with them (and yeah, I know, issue of scale, even a group of people dedicated to ruining the effort would be vastly outnumbered by background reCAPTCHA traffic).