W hen I was guiding and outfitting in NE, we had a problem with them. They travel on a wide circle. When we would see prints in the creek bottoms, the deer vanished and there was no use putting a huntrin there. After a few days, the deer would come back and we would be fine for a couple weeks.
Never saw one, just plenty of sign. I all my years in the wild, I have only seen three, one in Canada and two in WY.
I have no doubt they are all down the major river chains and have been for some time. As to a breeding population, that I don't know about.