This is just a curiosity thing for discussion and perhaps someone is involved in this. At some pipeline road crossings where the warning signs are located, I have seen 2" aluminum caps. At one particular location there were two different companies. Neither monuments were center punched, so am curious if these are low order markers or what purpose they actually serve.
I have found similar pipeline monuments.
They were the type with an iron rusted away setscrew and lose on a rebar.
Were found in the vicinity of a boundary line.
I've worked pipeline before and I'd call these low order marks. They appear to be marking locations above the pipe that are used during the pipe inspection process. We've set hubs (no tack) for this process before. This would be the first time I saw a pipe and cap. The marks are as reference points to track the process of the pig ( https://en.wikipedia.org/wiki/Pigging ) during its use. Usually there is a GPS enable sensor that you would place at the mark.