My sister once asked me how I can spot birds and animals in a woodland, as she found it impossible to see any signs of life whatsoever. My answer was that if you go crashing through the undergrowth throwing a stick to your dog, or even talking to someone, then you will see little. If you go warily though or sit quietly, you will hear birdcalls and movements in the trees and might even be lucky enough to have something like a weasel come to investigate you, or happen upon an otherwise engaged badger.
A mate and I once had the privilege of being peed on by a red squirrel for sitting under its tree!
Something kept pace with me one night when passing through the Etive woodlands. On one occasion I turned with my head-torch and saw yellow eyes. I have no idea what it was and, to be honest, at the time didn't really want to find out. My response was to turn back towards it and make a lot of noise, thumping a stick on the trees. My next priority was to get myself out of the woods and get a fire going.
I've also had the delight of being followed for an entire day on the Cairngorm/MacDhui plateau by a solitary reindeer who just seemed happy with the company. It didn't want anything from me and wasn't interested in any goodies I offered. But wouldn't it have been an entirely different matter had this happened in darkness and I couldn't identify my companion?
Fear of the unknown or an unfamiliar environment has strange effects on us, especially if the senses we most often rely on are taken away.
It's undeniable that those who go off into the mountains and other remote environments, particularly when alone, experience odd things. Sometimes there is an explanation... like when a mate of mine caught a glimpse of a strange, featureless white face looking in through a bothy window. It turned out an hour later to be a white garron, a type of horse used for bringing stags down of the hill, but I can easily picture him sitting with his back to the wall staring at the window and listening for noises, fully in the knowledge that the nearest habitation was 9 miles away. I've been in similar circumstances. I've lain in a tent in the middle of nowhere after imagining a human whispering, knowing full well it's the stream a hundred yards away!
It's in circumstances where we can find no rational explanation that cause us to look for alternative reasons for what we've seen or heard or felt; the bumps and crashes at night in a bothy when you know there's no-one there; seeing minute lights flashing away deep down in an inaccessible ravine on a cold rainy night; the short hairs on the back of your neck standing on end for no apparent reason or the sense that you are not welcome here!
I spent quite a disturbing and rather noisy night in one notorious Scottish bothy which puzzled me for more than a decade. I got my explanation a few years ago when an earth-tremor was recorded on Loch Etive, the epicentre being in the immediate vicinity of the bothy!
http://forargyll.com/2009/04/loch-etive-earthquakes-provoke-concern-for-inveraray/
If you go out into the Scottish hills, and many other areas on the planet, often enough you will encounter odd phenomena. How you deal with this is up to the individual, and I know a few fairly hard nosed people who wouldn't entertain the idea of stravaiging off alone into the hills for a week. Some are simply more careful than I, some need companionship but there are others who have had some pretty bewildering experiences and don't care to repeat them!
Not everyone has an interest in the countryside common amongst those using this forum and I readily accept that most cat sightings are probably by someone walking their dog on a path. There are fake photos doing the rounds and there are those who would wish to profit in some way.
One long-term, seemingly dedicated Nessie Watcher, in his frustration, lost the plot completely and published photos of UFO's, which were quickly identified as frauds and roundly dismissed and all of the astute historical research he had done was binned with him. That man disappointed and embarrassed quite a lot of people who had taken him seriously.
The above, however, doesn't explain the farmers, forestry workers, nature wardens and others who have made claims of cat sightings!
Cheers.