Hey ArnhemAnt,
I'd be willing to bet it has a lot to do with the EMF given off by the alternator, spark plugs, etc. If it's not running then there's probably some permanent magnets in the motor somewhere too. (But you know all that.

)
I used to have a 6m inboard/outboard but downsized to a 5.2m. I've found that most of the lakes I like to fish around here don't allow gas powered craft. We also have some pretty "ugly" places to fish:
View attachment 4774
The H will only go up when the water is like you see above. (Sorry, no aerial shots from the H until this summer.) I doubt the on-board batteries are an issue and I'm hoping they aren't. I've got 8 deep cycles on board as I'm 100% electric powered. I deploy two solar panels at the bow that make a good launch/landing pad: 80" x 27". (Used them for the Q500) DC motors = permanent magnets. The bow motor will be deployed and off at launch so the magnet will be a good 5ft away and enclosed in a steel housing. Fingers crossed it won't be an issue. As all flying will be within a 5km range from camp, I'll perform all calibrations on shore. Then it will just be a matter of getting a good satellite lock before launch. I prefer to land as opposed to hand catch however, I like to be comfortable in the event it becomes necessary. Murphy's Law reference: "Murphy" loves to work overtime. I like to be prepared when he decides to shows up.
All good info... Thanks for sharing.