Lots of hand tools, usually wrenches. I suspect that they fall out of recently fixed cars, where the mechanic failed to put all their tools away.
One thing that started to appear in quantity last year were masks.
+1. I've also found dozens of hand tools over the years, mostly wrenches (SAE and metric) as you note but three hammers, three Vise-Grips, half a dozen screwdrivers, two tape measures, side cutters, lineman pliers, needle nose pliers, two folding knives and a Leatherman-style multi tool. Most are cheap tools but a couple were really good like a Snap On 18mm combination wrench.
I also expect they fell off of workman's trucks or out of cars.