He was not sure why he knew too, maybe because of their connection with the guardian. That serpent was once the owner of the land but it was subdued by those ancestors until it relented and chose to sleep rather than continue fighting. Since the ancestors wanted to keep the serpent under wraps, they formed a contract with it. That must be the reason he could sense the sleeping serpent.
"Because that place felt the same as usual," was his only answer before pulling and releasing the reins. He then used his feet to pat the horse's side to signal it to move.
The master looked at the duke's back before he too followed. He gave the place one last look before it fully disappeared behind the dense trees of the forest. Rain started to fall down again before they could reach the third place but the duke showed no stopping so Cygnus couldn't complain.