Why would "inside a cave" make an uncanny mark or design , of necessity, a treasure symbol ? Whether inside or outside of a cave, you will always find landscapes riddled with uncanny designs, symmetrical designs, etc..... I can look in my backyard and find squares (the paver stones) triangles (the birdfeeder base), turtles (the rock art in my gold fish pond), etc....
I can lie on my back, stare into the clouds. And ... if I concentrate, can eventually find a smiley face, or a bunny, etc.... by the shape of the clouds. Don't over-think it. Wherever man-kind has ever been, he's left graffiti, designs, marks, etc.... And they can mean absolutely nothing at all except graffiti, designs, marks, that don't necessarily mean "treasure".