Found It!

Found the source of the 'Door Ajar' problem. It wasn't the sliding door in my case, Bonjour Tristesse, it's the front passenger door. It isn't loose in the lock but the sensor disengages sporadically. Fortunately, this means I don't have to accept the klaxon. If I open and and shut the door (in a safely parked location, naturally) the light goes off. For a while, at any rate. I'm going to look into shorting the sensor.