The brake warning light on my 2010 Grand Caravan is on, and I can't figure out why. Neither the ABS nor ESP/BAS lights are on. Is there a cost-effective way of figuring out which sensor is causing the light to go on?

The light comes on with a single ding about 15 seconds after putting the key in the on position.

Things I've done :

Tested the brake fluid resistance with a multimeter. It goes up to >5M Ohm so I don't think the fluid is bad. I've also flushed a ton of fluid through bleeding the brakes.
Disconnected the brake fluid level sensor. This makes no difference.
Disconnected the brake boost sensor. This makes the entire system flip out and beep like crazy as well as putting the ABS and ESP/BAS lights on. That suggests to me that it's not that sensor, although it is possible that the behavior for bad data vs no data on that sensor is different. Still doesn't make sense that it could trip a light with the car off when there's no vacuum to begin with.
Ruled out the parking brake switch. Pushing the parking brake causes the brake warning light to flash when in motion, so it can't be that switch.
Ruled out the brake pedal switch. Pressing the brake lights up the rear brake lights, otherwise they're off, so it can't be that switch.

My suspicion is that the brake fluid level sensor is bad, but thanks to the Chrysler standard "do as poor a job as imaginable, then figure out a way to do it worse" engineering, there's no way to get the sensor out of the reservoir, and it may not even be possible to get the reservoir off without breaking the non-standard bolt/pin that connects it to the booster, and I don't want to start breaking and replacing stuff just to see if it's the problem. Is there a way to actually test the sensor, some way to electrically test the system, or a relatively inexpensive scan tool that can probe the brake system and find out what is triggering the light?

Thanks in advance for any help.