My car did this, tracked it down to what seems to be a glitch in the ignition system. What started it for me was pressing the ignition button without holding the clutch, to turn the ignition on only (I think I needed to close the sun roof or mess with something in Sync). After that, every time I opened a door, didn't matter which door or if it was locked first, I'd get the chime like the key was in the ignition. The ignition acted normally, you'd just hear the chime.
I've read about other having the issue and it goes away on it's own. I've only had the car for about a week and a half (brand new) and it did it for about a solid week before I got really annoyed by it.
I was able to make it stop by pulling the negative battery cable for 30 seconds. After that, it stopped doing it. Pulling the cable only resets the trip odometer and it's other functions (average speed, avg mpg, etc), the ECU didn't reset not did anything in Sync reset.