It's possible. All graves are not marked for one reason or another.
I designed a water system that went through a small town called McCroy, AR. I also did the surveying on the water system and had located the highway right-of- way. As the construction company was approaching an intersection, I discovered that there was a cemetery there. There was a square monument that I thought was a highway right-of-way monument. That monument turned out to be the only head stone left standing in the whole cemetery. People have been driving over his grave for decades apparently without even noticing it.
The back hole was 100' from the grave of the guy that the cemetery was named after. I had to stop the construction and re-design the water line. It cost several thousands of dollars in 8"couplings,pipe, concrete blocking, two highway cuts, valves and labor.Can you guess who's fault it was. That's OK I took it out of the contractor's hide. He owed it to me anyway.
HH,