It’s normal to experience occasionally, especially if it’s fleeting or you’ve recently seen a horror film. It wouldn’t be normal if it was happening every day or multiple times per week. In that case, you’d probably want to check in with a therapist to get screened for psych issues or maybe identify whether stress in another area of your life is causing it.

Go see a doctor, that is not normal

It could be anything from diet through to illness and Lemmy is unlikely to provide any useful advice until you find out

That's just your FBI Guy.

Jokes aside, I would go see a doctor.

Get a carbon monoxide test done for your home.

I'm sorry. I'll be more discreet next time

Not abnormal unless it happens all the time. In my own life, noticed a similar feeling if I've been under stress for a prolonged period of time.

You have nice hair today.

It's certainly normal for my long-hair having ass. Sometimes I see a hair in my peripheral and I think it's a person, so I look and there's nothing there.

I have bipolar disorder and that exact sensation of seeing someone out of the corner of my eyes is something I get when I'm off my meds. You should consider seeing a psychiatrist

I had this a lot as a kid when walking alone or home alone. I wanna chalk it up to general anxiety/paranoia and how your brain decides to manifest that.

You don't have to answer here but consider your life. Did you grow up feeling unsafe? Do you have general anxiety and has it gotten worse as of late? Any increase in stress?

Something that helped me is owning my space, confronting the thoughts head on. Feel like someone is in your house? Call out and stomp around searching. Tell yourself out loud there is nothing there or perhaps play the TV/music and fill space with sound.

Last idea, get a cat or dog. They'll notice if someone is around.

My phone watches me all day.

It's perfectly normal to a certain degree, especially if you're home alone or at night when everyone else is asleep.

Does it usually happen after watching a certain type of movie? Like a scary movie or a home invasion movie or something like that?

Did you have any past trauma? Were you previously assaulted in any way?

But if it's so often that it affects your sleep or quality of life in general it could be some form of anxiety. Can you provide more details if possible?

I have that feeling most of the time.

I've been paying a private investigator $100/month to randomly spy on me. That way when I get that feeling, I know it's just him, and I don't have to worry about it.

Check inside your roof and walls for campers. Place some door chimes around, inspect roof hatches for use. Use tape and tack some thread to doors, if the thread/tape is disturbed the door has been used.

Exercise more, have some probiotics, these reduce paranoia. Masturbate less, masturbation contributes to neurotic mindsets/thinking.

Relax, try to meditate on being comfortable with your own mind (no devices). Go outside, maybe for a long walk. Consume skeptical media that debunks things. Become an atheist. Understand you're mostly alone, and just not a reliable narrator.

In my job I have worked with equipment that can use your heat to see through your house and use your windows and a laser to hear inside your home. So someone may be warching.