Test probe? What sort of test probe? There are so many devices available now I cannot be sure what you were using but it sounds to me more like it was test continuity or something. You said you got a light with the lead disconnected so it must be because it would not be detecting voltage, and you would like to think there was power to your fusebox.
To be honest, you are wasting your time doing anything until you have measured current draw with engine and all electrics off and in the locked/alarmed condition.
Get your multimeter, look for A and beside it a solid line and a dashed line below it, this indicates Amps DC - most multimeters will do amps DC.
Now to select a value, the meter will probably have a fused and unfused setting, if you really have no clue what the reading might be and suspect it will be quite low then always use the fused values - worst case here is you blow the 1penny internal fuse, if you misjudge something and you fire over 10amps through the unfused setting, chances are you will need a new meter. I am positive your draw will be less than 10amps, but it could be higher than 200mA and could blow the fuse, so turn it to the biggest value - 10A I expect. Pull one of the leads off the battery, say positive, and put the red lead to the POS terminal and the black lead to the positive cable clamp - ALL the vehicles electrics are now running via your multimeter - so don't turn things on!
Now you need to recreate the condition that is you leaving it overnight, does it have an alarm; late 1994 300tdi model? if so, hold down the bonnet switch if it has one, and lock the vehicle and set the alarm (this operation will still be within 10amps). It will settle down and should draw less than 30mA.
My 300tdi Discovery draws about 25mA in the locked/alarmed position - so every 40 hours it's used 1Ah from the battery or 8.4Ah every 14 days... Which the 85Ah battery copes with no problem - although, I wouldn't like to leave it alone for 3 months or anything!
Now - say you get a large reading, you now want to work out what it could be, if it was the alarm for example, it should go away when you unlock/alarm it. If not then it is something else, now keeping the meter connected (someone to look at the screen for you might help now) I would go into the fusebox and pull fuses for things like the radio, lighting circuits (dodgy relays can sap power) everything really untill with any luck the parasitic load drops off.
Try all that first and see how you get on.