Next test leave the battery hooked up but disable the glow plug circuit. There is a small chance the glow plug circuit is self energizing periodically. Just restore the glow plug circuit when starting the car and disable it right after the car has started.

You want to find the cause as quickly as possible. Drawing a battery down repeat times seems to damage them. Or shorten their service life. To me the modern car battery has low tolerance for being heavily discharged.