To start with, you might not have an accurate scale.
I am a real diet Nazi, and lose 1.5 lbs. overnight every night. My scale displays weight to the 1/10th pound, and I have been using it every day for the past few years, that's how I know how much I lose at night.
Remember, if you can fog a mirror, that is just part of the water in every breath you exhale. Besides if you get up and pee once, you can see how it adds up pretty fast.