What is this in the sidewalk?

Yes it looks like the freemasonry square and compass symbol.. Looks like someone has stamped or ground it into the sidewalk .. It probably was put there by a person who owned a business near it as a symbol to other Freemasons to support their business.. It looks like there may have been an insert that has been ripped up..

i have no clue what that would be. maybe if i dropped acid or took shrooms i'd have a better idea.

Yes, a sign of some sort... if you look closely, it's slightly off (the upper piece tilted to the right) and a Free Mason insignia would never be made that haphazardly.

my first thought was areemason, could also be an old survey marker

That was the first that came to my mind too :D 🖖 !

The hoofprint of the devil

Free mason sign 🤔

looks masonic.

Two pieces of metal embedded in the concrete, the cut off (with all the crap which finds its way into such a hole left behind).

I think you are right: right-angle piece held the sign, the other piece likely was installed to do any/some of a few candidate jobs (protect the sign from sidewalk scrapers/cleaners; reduce the wind drag from the direction where the curved piece points; etc.).

I *do* like the Freemasonry notion! lol

Star Trek combadge.