I always wonder how they can be so sure what these things are. In this case, for example, it's described as a "carved grid" and "is hewn into the stone slightly clumsily". Perhaps it was made for storing some specific small item? Or maybe it was used to keep track of something (i.e. move the pebble to the next grid space every time X happens). Or maybe it was just someone bored and carving a design?