Check the interior lights, glove-box light, boot light and engine-bay light (if fitted) to be off - consider removing the bulbs to see if that solves the problem.
To flatten a car battery over night, the drain must me at least 1A. If you have access to a multimeter – disconnect the battery negative terminal and place the meter in circuit, measuring the DC current flow, with everything off. Then pull the fuses one at a time, to see which circuit is causing the faulty current draw.
I have in the past encountered a faulty alternator to be the current path – which was intermittent, dependent on where the alternator carbon brushes happened to come to rest.