In watching a documentary on PBS's Nature about Coywolves (coyote-wolf hybrids that are emerging as a possible new species in eastern North America), I was intrigued by the fact that they are extremely intelligent and elusive animals. In fact, the documentary went to great lengths to impress upon the viewer that these animals go virtually undetected in densely populated urban areas--including New York City. This is largely because these animals become acclimated to the rhythms of human beings and make a point not to be seen. They will frequently wait until the coast is clear before moving on again.
I bring all this up because it establishes a precedent for an animal in North America to remain undetected if it so chooses. If it weren't for the sheer (and increasing) numbers of Coyotes and Coywolves in recent decades, I daresay few would ever catch glimpse of them.
Now, if you were to take an even more intelligent animal, as one presumes a Sasquatch-like creature to be, and add to that a population that would be substantially smaller, you've got a good recipe for its mythic status.
I think Sasquatch population estimates (flawed as they are) over-inflate the data. If such a large creature with the attendant attributes of high intelligence, strength, and possibly long lifespan were to exist, would not the breeding be kept to a minimum? Large animals usually don't have litters of offspring. And if a Sasquatch-like creature is more like humans, then they aren't likely to have many over their entire long lifespan. They would possess the wherewithal to survive as a species by other means than a genetic deluge.
This is all to point out that intelligent animals are aware of us and the threat we pose and can go to great lengths to avoid us even when they are in our own backyards. So, is it really that big of a stretch to think some hirsute hominid could be doing the same?