Sticking detent on ignition switch
 

A few weeks ago the key fob on my 2006 6 wouldn't lock or unlock the doors. I thought at first it was a dead battery, but found that pressing the unlock button on the fob would cause the door lock to click and reset itself. I concluded that the ignition switch was bad, but a mechanic at the dealer told me that the problem was that the lubricant on a detent in the switch had gummed up so that the switch was acting as if the key was still in the ignition. It took about 4 days of repeated injections of lock deicer and graphite to clear up the problem. I was told that in some cases the detent cannot be degummed and the switch has to be replaced.