I started getting this message last weekend. I don't keep a key in or near the car and it's 6 MT do can't be those issues. I also do > 12k miles / year so can't be lack of miles.
Last week I did have my car's right side sound cut out - so maybe something has gone amiss w/ the stereo, but bottom line it seems to me I've got a battery problem.
Guess I'll be shooting into BMW before I get stuck