Well, there have been many reports of "finding" sasquatch nests -- I guess the difference between that and finding a cougar den is that we do now know what a cougar den looks like...and I guess it's easier to believe people who say they found a cougar den v. those who say they found a sasquatch nest...now that said, I don't believe the governement study mentioned anything about a den.
And while we're talking about credibility of eyewitnesses -- almost all reports of cougar sightings have turned out to be dogs, fishers, house cats etc. and then there is the persistent rumour of how the government (never specific which level) has released cougars to manage the deer population or the turkey population etc....everyone knows someone who knows someone who came across an MNR p/u with a cannister trailer behind it and a cougar inside...
As for detection, I guess that's part of the argument as to why sasquatch, like the cougar, is so seldom seen -- it can detect us before we know they're there.