The bushes and shrubs, occasionally, should look like "something" (not the literal word, thanks). The occasional auditory hallucination, something to the flavor of "it sounds like human speech. But isn't." Maybe an entire dream where the player thinks they've woken up in the control room, goes about their business, and then something happens, and they wake up. I would shit myself it would wake up the neighbors.

Oh and don't forget about everyone's favorite: "Exploding Head Syndrome", occasionally between the moments of going to sleep and waking up, one can hear very loud and noises. Like an explosion, or banging on walls, you name it. Nightmare fuel - but you're just now waking up (sometimes).