We have clear evidence that some ancient sites were conceived or planned long before they were built. And that the building of the greater site took hundreds and sometimes thousands of years.
Given that humans and their cultures are fragile and mortal, there is great potential for a site to end up incomplete.
Graham Hancock seems to have speculated (and didn’t go much further) that the Giza pyramids were meant to be more numerous, and would have been an indisputable match for the stars of Orion when finished, instead of just the belt.
We could imagine the missing pieces and look for evidence, which most likely is just a dolmen, pole or hole in the ground, as a place marker.