You wrote of a chap whose batteries ran down after three idle days and gave him a possible theory. Actually, the matter is simple to prove - to anyone with any electrical knowledge. You switch off everything as if you are leaving the car in a normal manner - but put an amp meter (ammeter) in circuit between the battery and the main lead. To clarify, disconnect the battery positive lead, put a meter between the two disconnections and if you read any current flow you have something still switched on.

How serious this is can be calculated. If you are leaking 1 amp and you have a 75 amp/hour battery then it will be totally flat in 75 hours - and unable to turn an engine over in about 30 hours! So a small "leak" can be serious - find the cause. For example, a 10 watt bulb left on will leak about 0.8 amps.