No mention of it in the handbook, assume it's like the engine oil level and no dipstick, being hands clean and non technical, BMW owners are not expected to investigate what happens under the bonnet, wonder they don't seal it down . A serious search all around the engine compartment elicited no visual evidence of said reservoir . However I finally found where the Bavarians had secreted it . Not much wasted space in there.
Stechen Sie nicht mit den Fingern in die Macinery und mischen Sie sich nicht in die technischen Details/
