Water loss that is difficult to detect and measure is called: sensible perspiration, insensible perspiration, negative water balance or diuresis

The correct answer is : Insensible perspiration

It's the water that evaporate from your body before it is perceived as moisture on the skin, usually happened inside your lungs

Since the water is gone inside your lungs, it will be very difficult for you to feel and measure it

You can literally find this on google of each science career and search up what type of training you need and how many years in school