The dreaded "Heebie Jeebies". I've had two notable events.
I was out bowhunting and got a very early start. I still-hunt (walk slowly on the ground as opposed to blinds or tree stands) and I like to be in place and in the woods well before first light. One morning I just had a odd feeling. The "hairs on the neck & gooseflesh", etc. I was eyeballing carefully and, about 20 feet in front of me and maybe 8 ft off the ground, this fierce pair of evil, yellow eyes glaring at me. I about soiled myself.
Big 'ol horned owl, probably pissed that I messed up his day's breakfast. He stayed around another half hour . . . and by then I was breathing about normal again.
Second was when I was snorkeling in Cayuga Lake. Maybe 30 miles long, 600 ft deep and more in spots. History there goes back to F&I War & long prior. We had anchored and swum to shore, and decided to spend the night. I wanted to make sure the anchor was set, and being already wet, figured I'd dive down and check it. 25 feet, which is as deep as I would normally snorkel but not abnormal. I took a lungful and surface dived, and as I got to where I could see the anchor well the light dropped suddenly and I got the worse case of "dreads" I can ever remember.
Looking back it's possible I passed through a thermocline and the temperature and zooplankton was actually colder, denser and allowing less light, but it sure didn't seem explainable at the time. We ended up moving the boat. Later, relating the story, I was told that a passenger steamer had a boiler explosion and fire and sunk near that spot with loss of life before the turn of the last century. I'm skeptical, but that day I wasn't.
Depending on your religious beliefs it is explainable in that we are genetically predispositioned to be able to subconsciously register a set of staring eyes as a danger signal and react strongly to it. The ones who freaked and got back up in the trees first were the gene pool of future generations. I believe we "feel" as set of eyes on us from visual clues just outsice of conscious observation. Them portrats with the eyes that follow you, for instance. We're hard wired to be bothered by that. I know from bowhunting that you never stare at a deer. Even in full camo they'll see/sense your stare and get antsy.