Even before the Summoning, Sylas' memory was on a completely different level compared to others. With his Wisdom increasing to its current point, it was only more exaggerated.
He could remember the exact configuration of the tattoos on Ragnar's body, even the ones not readily visible through his clothing. That was because when he almost killed Ragnar through the Scorned Wraps, he had sensed the clear fluctuation of the tattoos, or rather, what Nosphaleen called Rune Etchings.
Because of that, even in the darkness, he had been able to feel some of the same fluctuations coming from this statue. When he felt them with his palms, he was even more certain of the similarities.
He noticed that Ragnar had made mistakes in several locations, and it shocked him that he was actually able to get his tattoos to work with so many flaws built up over them.