I guess we are talking about ghosts(?)
Some of the reports that I've read mention a vision or sighting of a thing, that is silent and extremely fast and agile.
In some case it's loud and in others it is more silent than the resident white tail deer.
So the question might be, is it of a corporeal nature or not?
"Paranormal" people claim there is something there..... When will we learn the truth?
The excellent observations made in your post indicate why bigfoot is studied by cryptozoologists.
I too am very, very curious about these creatures. Too many have been sighted by reputable people with no reason to lie to not believe that there is not something out there...whether it can be proven or not.