It takes her a good while to figure out he can turn human, mostly because McCree has to shift out of necessity.
During a leisure trip to the nearby stream in the forest for a drink of water, Sombra enjoys watching McCree drink from it. She’s not shy about her fascination with the dragon, which McCree is sort of disgruntled/embarrassed about now that she isn’t just strictly terrified of him.
People usually don’t wander into the woods beside the mountain for many good reasons, but occasional they’ll hurry on through. This happened then, with McCree hearing the humans before they could spy them through the trees. Without any real place to hide, and not wanting to fly and be seen, McCree shifts into his human form.
This stuns Sombra into near silence to see the seemingly ordinary man that the legendary monster became, but she recovers just as the travelers see them. McCree doesn’t look directly at Sombra until she grabs his hand and slips against his side, as if they’re down that thousands of times before. He doesn’t say anything as Sombra converse with the travelers, saying that she and her husband were just on a stroll but they shoulder hurry. This place isn’t safe.
They do hurry along. The moment they are gone, Sombra steps away and drops his hand just to stare at him. With near whiplash from the last few seconds, McCree grumbles about her rudely staring. Sombra comments on how she didn’t know he could do that. She also tells him he owes her one, for few would believe a woman and man without any relations to be out in the woods for no good reason.
McCree huffs, before saying a strange thank you that Sombra takes a sincerity.