While living in Hawaii, the wad a place that I passed between Makakilo and Barber's Point.
I'd pass by one day and rocks would be stacked on a boulder. A daddy or two later they would be gone. Then they'd be back and gone again. This went on for at least two years.
The thing is that this area was on the other side of a fence from Kamehameha Highway with no apparent access.
Interesting experience!