what if the first two piles you weigh don't balance?

If they are plus coins, the heavier one is the imposter.If they are marked with minus,it's the lighter one.But what if the first two piles you weigh don't balance?Mark the coins on the heavier side with a plus and those on the lighter side with a minus.You can also mark the remaining four coins with zeros since you know the fake one is already somewhere on the scale.