I can't believe nobody has noticed this yet!

Sometime in the first 45 minutes of the film, Arnie's eyebrows just disappear completely with no explanation at all. Trust me, watch the film. When he first appears he has his usual dark, bushy eyebrows. But by the time he is driving around the carpark looking for Sarah Connor, he doesn't have any at all (which is why he looks so bizarre).

I think James Cameron did this because it was easier to make a dummy special-effects head (used later in the film when Arnie performs eye surgery on himself) without having to put realistic eyebrows on it. Either that, or the explanation was left on the cutting-room floor.