The alternator regulator discharges the battery, albeit by a very minor amount.
You will notice a vary small spark as you connect the battery leads.
The way to prove what's causing it is to put an ammeter (multimeter) in current (amps) mode.
Connect the -ve cable of the van to the battery.Leave the +ve lead off.
Connect one lead of the meter on the battery terminal and the other on the positive lead.This should show the discharge.If you get a negative reading on the multimeter reverse the leads from the meter.
Systematically disconnect all electrical equipment.
It sounds as if it's an alarm or demobiliser.
You haven't left the 12volt fridge on have you?
You did not give a time scale that the battery discharges over.