Ok, do this exercise, right now.
Concentrate, look infront of you, try to hear whats around you, be sure to be alone. You will hear something. This is your brain, trying to catch a sound that isnt familiar. Tree branches sounds are familiar, you don't really hear them, when you concentrate, you hear a lot more noises, and you also are thinking about something weird happening, so you think that these noises are spirits of something, when they actually are a natural noise that your brain makes you react as if it was something dangerous.
Again, look forward, and try to see whats on the side of your eyesight, trust me, you will imaginate a lot of stuff, and you will eventually turn around and look.
Tried it myself, I thought I saw someone at the window, I could even see a face and a cape. I looked afterwards, and it was simply the reflection of my cat with the shadow of the fence.
tl;dr your brain is imaginating stuff, and out of fear, you don't think and assume its something.