Since I've been doing some walking in the wilds by myself for a few years, I've had a couple of moments where the hackles rise.
I always follow those reactions, and move away to another area down the track a bit.
The last time that feeling hit me, I was walking alone in the Mackay Highlands. It was rough terrain, the vegetation was pretty tight, being less than 50m for most of the day.
I'd pulled up for the afternoon, deciding to setup for the night. The area was a bit more open, with visibility opening up to 75-100m.
Old habits die hard, and I still stop under a tree or in shadow whenever halted. It's a concealment thing, and I was a slow learner as a young bloke, with grumpy old men beating the lesson into me via boot and pacestick

Anyway, the hackles rose on the back of my neck. I definitely had the feeling I was being watched, and that "they" were on my backtrail. Normally, I stay fairly low out scrub. This time, I actually stood, looking at the treeline over 75m away.
For some unknown reason, without conscious thought I also had my bush knife in my hand, in a fighting crouch.
Felt very uncomfortable after that, so packed up my gear and walked a couple of klicks to another spot. Felt much better after putting in a fish-hook and some distance from that spot.