Well, this got more interesting than I’d thought!
Re Philadelphia: I was born north of NYC, grew up in Ontario, moved to Virginia 35 years ago. So on average, I guess that puts me in Philly but I’ve used “pavement” to mean “the hard stuff, wherever it is” and never had any confusion, so I don’t know what to tell ya.
These are JUST nails, heads smaller than a roofing nail. So maybe not surveying marks–when I saw that, I was thinking more like temporary marks.
Mystery…perhaps continues? I don’t think it’s kids, as there are too many spread over too wide an area (we have many miles of paved trails in our subdivision and I’ve seen these miles apart, as well as on concrete, where I doubt kids would be able to embed one).