When does a battery self-discharge?
Battery self-discharge is caused by:


leakage current flowing internally INSIDE the battery, you cannot measure it !
chemical processes inside the battery causing loss of the amount of energy stored in the battery 


A way to measure this could be to:


fully charge the battery
disconnect from charger
wait until the battery voltage has reached it's lowest value (when the battery is empty), note that could take a YEAR (on a lead-acid battery) !