Hey,
I had a similar problem a few years ago Brand new home, 4 years old. After the first 2 years of ownership, my neighbors and I agreed to put up fences and split the cost. I did all the fence and they contributed to half the cost of the material.
When it came to the property markers, the rented metal detector could not find anything at all. We used a 200' tape measure to estimate where it should be from the property plans and still couldn't find it. We even dug around a few inches to see if we could hit it and had no luck.
Eventually, we reached out to the builder since they were still building in the area (another phase) and they sent the site supervisor out. He also took measurements but after 60 minutes, couldn't find it.
The next day after work, I went to look in the back yard and there were bright orange florescent X's sprayed onto the grass. The metal detector still couldn't find anything. When I call the site supervisor, he said he had someone come out, they couldn't find it either but they measured using the plan drawings and then marked it.
According to the measurements, the X's were in the right place so my neighbors and I were OK with it. The supervisors assessment was that with the type of ground we have - very, very rocky as we are built on an old quarry - it's possible the rebar/rod they used ended up sinking further into the ground with all the heavy construction traffic around.