Wherever there is a connection in the pipeline there may be a leak. Clean the tube, then use a dry paper towel or absorbent fabric to check for moisture in the tube. The shut-off valve is often the culprit without being noticed. “A small drip can occur. However, the shut-off valve has a nut that can be tightened by compressing the seal under it and down around the stem sufficient to stop a drip. ”
In homes over 20 years of age, it may be that during a change of a faucet or a mixer to install a new one, the rubber seal that prevents water from passing will break “the pieces could clog faucets from other areas of the faucet. Home.”
Check the sliding union nuts and washers under the siphon sink. These may leak over time.
If none of these suggestions works for you, you may need the services of a professional plumber with ultrasonic detection equipment and / or telescopic cameras who will verify possible suspect areas and will determine with a high (but not infallible) accuracy wherever it occurs loss.