Why the door ajar alert will not go off when all the doors are closed?
Probably a bad switch in the door.
My Ford does it occassionally and it's the door switch like tenspeed mentioned. They tend to get dirty, so spray some WD-40 liberally into the door latch mechanism on one door to free the switch up. If the light still doesn't go out, proceed to the other doors until you clean the one that's causing the issue.
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