Do I really have to mention that canvas or whatever material they used to do paintings was highly valued (rare) back then, so the concept of someone reusing a canvas was common?!! If the image is hidden, it’s likely because some rich guys kid made a mess and someone covered it over to do real art!
I think this is on a cave tomb wall.....................
I think they’ve been using some sort of x-ray analysis of old paintings to uncover previous work beneath the visible finished one. It usually seems to be earlier versions of the finished paintings, as opposed to entirely different ones.