Malfunction light comes on intermittently?

I have a Nissan GU Patrol 1998 with the 2.8 diesel. Had it since new and covered 300k of faultless running.
Now on a trip around Oz towing a camper trailer.
1 week in and found the malfunction dash light symbol has come on for a few hours and when stooped with engine switched off has not reappeared again!
It came on briefly at the very beginning of the day but went off when I shut down the motor.
The only recent modification to the vehicle recently prior to the trip was the installation of a 2nd battery.
Could this be affecting the computer as it charges through the Anderson plug to the trailer battery?
Has anyone experienced this problem and if so what was the cause?
The engine is running beautifully so I don't believe there is anything wrong.
Cheers
Pete