People are more scared of what they don’t see, as opposed to what they do see.
If you can create a creepy enough environment, and allow the main scare/enemy to remain anonymous. Then the human imagination takes over and can create insane illusions of what the enemy could be.
Also another scary method is blood leaking out of an opening in an overhead vent. The players curiosity is to find what is bleeding. But at the same time they are scared to approach incase it could be an enemy, or even a jump-scare moment.
Overall the best thing to do is be unpredictable. Make the player expect something, then deliver something totally different.
Players are constantly expecting jump-scares, and players like to assume places they’ve visited recently are safe to visit again.
Watch a lot of horror movies as well. Insidious is a good one, ignore the jump-scares and concentrate on the parts which actually creep you out. The main culprit in Insidious is invisible and left to the human imagination for the first half of the film, and that makes it really scary.
There is also a part where a man is walking in patrol outside a the window, and then suddenly in line with his moving he shifts through the wall, at which point the female observing him screams the man realizes she’s there and attempts to attack her, she closes her eyes in fear, opens them moments later and he’s gone.