It may be just coincidence that the drain happend at the same time as the car locked mate,my money would be on the heater resistor although the door locks can cause this dont let them rip the hole out of you for checking the car. you need to dissconnect the door locks one by one & see if the car goes to sleep. Best way to save yourself cash is take the keys out the ignition & if the heater is still on or fans still work when keys out its the resistor.
Let me know how you get on even by pm