Thanks for the good word, BMWEnthus. As the problem seems to be related to Fuse 21 only I would be going for everything connected to that circuit big time. Do you turn the radio down or fully off? If you used to turn it off but now only down B4 taking the key out it could be that the radio was put on permanent power in error - just a thought. I guess that if fuse 21 is pulled the battery stays good for a week (or at least several days)? If this is the case you will have to isolate the items on the fuse one at at time. Slow I know but when you find the faulty unit there will be no doubt and it is cost free.
Battery terminal/wire and connection corrosion should be fixed asap. A strong solution of bi carbonate of soda in hot water will nuturalise the acid growths and then clean and test/inspect all joints, terminals and earths.
Globe/bulb removal will stop any drain from a light, again check for corrosion. How is your charging voltage?
Good - Fast - Cheap: Pick any two.
E23 735i; E32 735iL (X 2 - 1 Alpine White and 1 Glacier Blue); E34 535i; E38 735iL; R50 Cooper; R55 Cooper Clubman.
E21 318i; E30 318i; E32 735iL; E34 535i; E38 730iL; E53 4.4i