In Chapter 132, Dora performed psychometry on a coin she found in an ancient campsite tucked away in a mausoleum in the Plaza of Bones. It was a silver coin minted in Ustalav in 4717, but seemed to be at least a century old:
She saw Rony/the Triangle Man sitting at the campsite by himself, apparently at the time when the party did the Dream Quests their first time. He still had the spiral tattoo of equilateral triangles at the time, and resembled what Gull saw when he looked in the mirror in the laundry room at Briarstone Asylum in Chapter 7.
He was sitting quietly at a campfire, quietly contemplating the coin. And then he specifically leaned over and set the coin in the corner of the mausoleum. The coin hadn’t been lost–it had been deliberately placed there.
And the vision faded.
First Reference: Chapter 132
Other Notable References: Chapter 143, Chapter 150 (Part 3)