Woof, this can be such a pain. One of the more common ones is the pump-out hatch for a septic system. In my experience, people usually do something like remember that it was on the cross-line between this tree and that tree, and that rock and this other tree. If you don't pump the septic as often as you're supposed to, the hole gets filled in, you forget, or maybe your markers have gotten shifted, and then it's miserable poking around to find that one particular one foot square spot.