Compass error - East is West, West is East, but North and South are North and South

System: 550 Hex, APM2.6 Flight Controller (supposedly with no onboard compass), 3DR GPS, latest Mission Planner, Hexacopter 3.3.1 firmware.

Problem: After calibrating the accelerometers, the compass, the radio, arming then dis-arming the motors, I switched Mission Planner to show flight details.  The HUD showed the front  of the 'copter was facing North.  Roll and pitch changes displayed properly, but when I checked yaw the compass moved in the opposite direction of rotation.  I pointed the front of the 'copter to the West, yet the HUD compass and the GPS  show it heading East.  

This also happened with this same hex when I used the same system set-up - but with a APM2.5 Flight Controller (which DOES have an on-board compass), the same Mission Planner, but with Hexacopter 2.3.1 firmware.  

A. I could see this happening IF the unit which houses the compass was mounted upside down - but that can't be the case because neither the APM2.5 Flight Controller (the one with the compass) nor the GPS unit (apparently also with a compass) are definitely NOT mounted upside down.

B. The fact that this happens with two different Flight Controllers - and with two different versions of firmware - Tells me it is not a Flight Controller issue nor a firmware issue.  

C. Magnetic interference - I've considered this but cannot conceive how it could produce the compass and GPS errors I am seeing.  

Anyone have any ideas here?

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