Waking up to find your own handwriting on a note you don't remember writing, with instructions you're already following. That right there gets my gears turning every time. Why would I obey myself? It implies a version of me that knows more, or maybe a version that's being controlled. Is it a message from the past or a warning from the future?
For something with more physical unease, try a character who discovers their shadow moves a fraction of a second after they do. It's subtle, maybe only noticeable under bright, specific light at first. The slow-burn horror of that, the questioning of your own perception, is a fantastic engine. The real suspense comes from the character trying to prove it to someone else, only for the delay to vanish whenever they're watched. Makes you paranoid on their behalf.
I once drafted a whole outline from the prompt: 'You are the town's most reliable narrator. Today, everyone else's story matches perfectly, and yours is the only one full of holes.' The suspense isn't about monsters, it's about social isolation and the terror of being the broken link in a seemingly perfect chain.