not trying to put Salt on the wound but remember I told you there has to be a leak somewhere, maybe too small to be detected by BMW (who just use a dye or sniff test, which only detects big leaks). freon doesn't just disappear over few month. anyways, BMW should change it for you for free, don't let them make you pay.
http://www.1addicts.com/forums/showthread.php?t=971381