Everytime we have guests onboard, they never turn the taps off properly, first we know about it is the pump going off.
I sympathise, my 10 year old nephew gave me a disbelieving look when I berated him for excessive use of the Porta-Potti flush - "No Uncle Alan, you are pulling my leg! You don't have to carry it to a disposal point and tip it out!"
I had a leaking Pressure Relief Valve (PRV). Often mounted on top of the calorifier (hot water tank) at the hot water outlet, its purpose is to relieve the pressure created by the water expanding as it is heated. Fortunately, the overflow from my PRV went to the engine bilge but I was still concerned by the amount of water appearing in the engine bilge in the 'dry season'.
On top of the PRV is a plastic knob. When rotated it 'clicks' up and down a little and a drop of water seeps out. Turning this knob re-seats the valve. This is necessary occasionally as the valve seat will be contaminated with hard-water deposits.
If you do not have a PRV you should have a diagphragm device to relieve the pressure. Often the same device as the 'accumulator' mentioned earlier but in the hot water supply circuit. If you have neither, excessive pressure when the water is heated may result in leaks from joints which are dry at normal pressures or, in the worst case, a burst calorifier.
In any case, when the water cools, e.g. you stop the engine or turn off the heating, the pressure slowly reduces and, eventually, the water pump runs to maintain the pressure. Probably after you are fast asleep. My boiler is controlled to only raise the hot water to 70C but running the engine can raise the hot water temperature to scalding in twenty minutes. Depending on the capacity of your calorifier this can result in a considerable 'leakage'.TB Training
explains everything about boat systems better than I do. Domestic Water Systems
may be particularly relevant.
Do not underestimate the cumulative effect of condensation; there may be hidden, poorly insulated cold-spots, e.g. under the gunwhales, that are dripping half a litre every night. After a few years the adhesive bond fails and the insulation is detached. The water runs over the insulation and arrives in the bilge (or on your pillow) some distance from its source.