There's no clear evidence of a puzzle: no raised edges, no odd bumps, no discolored areas. No color-changing tiles, even. But Prestidigitation does reveal one thing: when the light is shone directly on the mural, it reflects off of the silver paint, shining at a slightly unnatural oblique angle onto a wall that diverts from the dragon wall by approximately 40 degrees. Where the light hits that wall, it illuminates some manner of paint that wasn't previously visible. The shapes revealed by this light are two humanoid individuals, one tall and one small, in the same configuration and posture as the dragons on the original wall.

Angus catches up to Papyrus and looks from one wall to the other, contemplative. "That's funny, isn't it? The way they're the same in both. And the second ones are hidden, so they must be important."