Here's a reply from Robby Ackerman who replied by email, but I thought would be helpful to others. With his permission:
My experience is ... in addition to the items you checked, and the IMPORTANT
items is the brake fluid level which was okay, this light comes on if a wear
sensor in one of the pads touches the brake rotor. When the wire in the pad
first touches the rotor you have 10,000 to 20,000 miles left on the pads.
For some inexplicable reason the wear pad turns on the light in the rain
with my 123 and my neighbor's 123. Are there MB brake pads in the car? you
can visually inspect the pads to see how much wear is on them and if you
want to take the time you can unplug each one, a 10 sec job on each wheel,
and see what that does to your light. I'd cover the light with black elec
tape and go on your trip.