Several possibilities. Most likely: Rear heater core, rubber head gaskets, plastic coolant tubes, front heater core, radiator, water pump.  Pressurize the system to 15 psi and look for leaks overnight.  Make your own pump with a tire valve stem like this one: http://www.gowesty.com/product-details.php?v=&id=24081  You'll be amazed at what you find. 

Stuart

-----Original Message-----
From: Vanagon Mailing List [mailto:vanagon@gerry.vanagon.com] On Behalf Of John Rodgers
Sent: Thursday, December 08, 2016 7:49 PM
To: vanagon@GERRY.VANAGON.COM
Subject: Coolant light 2.1L WBX

Driving down the road, all needles where they should be, red coolant light
comes on. Stop the car, check the fill/overflow tank. It's up to the top
mark. Next , empty the baggage area, raise the engine hatch lid. Removed
the expansion tank top, found coolant to be below the sensing prongs on the
sensor, thus the flashing red light. Topped off the tank to get home. No
more red light. It was dark, so must wait to tomorrow when it cools off to
see where the coolant  went. But I could see there were no coolant leaks.
So the question is - where did the coolant go.

Ideas??

John