Did you know that you can be below 55% hydration and feel like you're overhydrated (peeing all the time)? Humans can have over 75% hydration levels. Think about this. A 250lb person can be about 125lbs of water or 200lbs of water. A 75lb difference. That's around 9 gallons. Your intuition, however, is run off of relative changes in recent history. So if you've been persistently and consistently dehydrated for a long time, you wouldn't even know it.
An often overlooked area of opportunity, some serious health concerns arise at even really low single digit percentage decreases in hydration. When people are feeling anxiety, lethargy, etc., in some cases it may simply be a persistent dehydration. Studies show that infants are at or above 75% hydration and the elderly and infirm are at or below 55%. Thinking that thirst or intuition will inform good water management in the body is folly.
Here's a great paper examining the variance found in tissue and implications with only minor shifts in relative hydration (a couple percentage points):