Stan wasn't sure when the scars first faded away.
They were there when Mabel was, that much was for sure. Mabel would ask him about them from time to time, which Stan had written off back then as a mixture of curiosity and lack of a filter, but maybe deep down she'd known that he needed to tell those tales, known that he needed to remember it all. But Mabel was long gone now, and so were Dipper and Ford and all the rest, passed through the reincarnation cycle more times than you could count on one hand, even a six-fingered one.
Around the point where he'd gone from counting the centuries to counting the millennia, perhaps. But he didn't know for sure- that was the whole problem, that he didn't know, didn't notice the blemishes that weren't there, didn't realize that anything was amiss.
There were, admittedly, periods where he didn't notice much of anything at all, periods where he let himself go, and maybe one of those had been when it started. At the very least, it was at the tail end of one of them when Stan first realized what was awry, when he glimpsed his own uncovered arm, smooth and pale and hairless, a vague approximation of the human limb it stood in for, and thought Well, that's not right.
The lack of hair was perhaps the most obvious problem; he remembered the kids ("the kids" had gone on to live good long lives, and what roamed the earth now included their descendants' descendants' descendants twenty times over, but Dipper and Mabel would always be kids to him) making comments about him having too much hair, and his arms grew bushy to match his memories, and wrinkled, too, like they had been that summer, the summer when everything had went to hell.
But even then, he could tell that something was still off, that there was something else-
That's when Stan remembered the scars.
The scars came back one by one, popping up on what passed for flesh as Stan remembered the stories behind them. The long, jagged line from a bar fight in East St. Louis that he had definitely lost, but that he still couldn't quite bring himself to regret. The little round one that was there from before he left New Jersey. The one from Columbia... and the other one from Columbia... wait, was there a third from Columbia? There was a third, come to think of it; Columbia hadn't exactly treated him kindly...
After his arms looked more or less right, Stan moved on to the rest, adding the scars back and almost getting lost in the stories they contained. The gash across his big toe, from when he'd nearly severed it working with some otherworldly metal or another while desperately trying to fix the portal. The little dots on his throat from threats never fully carried out. And of course, the scar on his right shoulder, a visible reminder of what might well have been the worst day of Stan's life, usually hidden behind clothes but still there... it had always been there...
How had he forgotten about that scar?
How had he forgotten any of them?
Was his human past really that far away now?
Stan would have sworn that the noise he made started out as a laugh, and perhaps it was for a moment, him laughing at his own folly. But the noise changed ever so slightly, laugh turned to sob, and by the time he had put back the last scar he could think of, Stan could feel hot silver tears running down his face.
With all of them back in place, he looked almost human.
Almost.