Every pumping crew in the county has spent an hour on a lawn looking for a lid. Most tanks here were buried a foot or two down in red clay with no marker, and the people who knew where it was sold the house years ago. Here is how it is found, from cheapest to most certain.
The county as-built
If the system was permitted, the Cabarrus Health Alliance file has a sketch with the tank measured off the house corners. It is not always accurate to the foot, but it puts the probe in the right ten square yards.
Follow the outlet
The main drain leaves the house on the side with the bathrooms, usually through the crawl space or the slab, and the tank sits ten to twenty feet along that line. A cleanout at the foundation shows the direction. Look for a rectangle of grass that greens first in spring or browns first in August.
Probe and locator
A steel probe finds the concrete top through a foot of clay in a few minutes once the direction is known. Where nothing shows, a transmitter flushed down the toilet and a receiver walked over the lawn finds the tank inlet. That is the locating charge on the price list, and it is paid once.
Then fit a riser
Once the lid is found, a riser to grade with a lockable lid ends the search for good. It is fitted on the same visit, and every pump-out and inspection after it starts without a shovel.
Questions on this
Can I find the tank with a metal detector?
Rarely. Concrete tanks have little metal, and the rebar reads no differently from a buried pipe. The probe and the transmitter are what the crew uses.