Hollllly crap. I shoulda known this since my Benz key did the same thing and I was pissed that my BMW key apparently forced me to have to physically undo my key from it when I gave the car to someone to work on.
I tried to find such a way to disconnect it, to no avail.
Kudos to BMW's design team for even designing a KEY so seamless and tight-fit that you can't see where it's supposed to come undone (Mercedes as usual a little more crude and behind the times in implementation.... ironically therefore making it easier to obviously see the actual key and how to un-click it
).