Methinks it's a lockout key of some sorts for mechanics when they work on your car.
I'd be a little wary though of trying the "what's this do ?" test. Pop it in where it fits and you may very well disconnect something !!
But, I have to admit, if you tried it and your next post is, "Every time I put my car in drive, it goes backwards", I'll be spitting coffee all over my keyboard !!