I will provide an honest answer. I trust this will be appreciated on a sunday. Please remember that a low rating PUNISHES me for being honest and that I receive nothing for my time until a positive rating is clicked.
normally if you get this warning the first step in diagnosis should be a scan for faults using a factory BMW diagnostic computer. The car has a lot of on board systems to help ferret out issues like this, and making a small investment in a proper diagnostic can save liiterally hours of manual testing trying to trace an initermittent issue that is not present.
if you do not wish to invest in proper testing, to test manually for an electrical drain you will need to - with the issue present - diconnect the vehicle batter and hook up an amp meter
then you will need to spoof the car so it thinks all doors and trunk/boot are closed, wait 30min for the car to go into sleep mode, then start pulling fuses one by one while monitoring amperage draw. whatever fuse stops the drain is the problem circuit
the most common causes are a faulty nav computer, fan controller or engine fan, but the issue could be ANYTHING on the car including the battery sensor
again, this is difficult testing to carry out, especially on this vehicle given its complexity. BMW has engineered computer diagnostics for this type of issue, and this would be the preferable route.
Please let me know if you have further questions. Please do not forget to click a positive rating as this is the only way I receive credit for my answer, thank you. Best wishes with your vehicle