Hi and welcome to the forum!
I would have thought a drain that serious would be caused by an intermittent wiring short.
By the way, just to get the terminology right (without being pedantic - sorry!), you are looking for a current drain, as measured in amps (or milliamps). Items such as alarms draw milliamps not millivolts. Putting a meter in series with the battery positive and the associated cable measures the current drain in amps. Ammeters in series measure current, voltmeters in parallel (e.g. across the battery) measure voltage. Never connect an ammeter in parallel!
Anyway, I digress. Connecting an ammeter in series with the battery positive and the associated cable is absolutely the right thing to do. I think I would be quite suspicious of things like door wiring looms, although obviously whatever is draining the battery is still powered when the ignition is off.
With the ignition off and the ammeter connected, try opening and closing the doors and the boot / hatch and look for surges in current. (It sounds as though you will be looking for several amps, so make sure the meter is man enough for the job). After opening and closing each door and boot, check that the current returns to it's normal quiescent level after all the interior lights have gone out. Also give the fusebox and relay box a bit of a wiggle / tap and see if you get any surges. It could be that the motion of the vehicle bouncing on the road is causing an intermittent wiring short. Maybe bounce the suspension a bit to see if you can create the problem. Be suspicious of anything recently fitted, such as the rear wiper motor. Did the fault exist before that work was done?
I would also be tempted to turn the ignition on and then off and measure the effect of that. You probably wouldn't be advised to attempt actually starting the engine with the ammeter connected unless it has been professionally connected, due to the massive current involved.
So in summary, connect the ammeter, open and close things to stress wiring looms and tap / thump any boxes such as fuse boxes which have the potential to be affected by vehicle motion. Consider whether recent work may have damaged something.
Hope you find something helpful in those comments! Good luck.