After my last near flyaway with compass errors, I thought about the issue a bit more. I have flown many many times without errors, and a couple of months ago a bought a pair of plastic extension legs that clip onto the arms. Looking at a log where I first got a speed error, it was about the time I fitted these before a flight, subsequent flights in the period after also show the odd speed or compass error, but only once serious enough to go into atti mode. Now, the last time a few days ago where I nearly lost my Spark, I had some problems clipping in one of the legs as they are quite a tight fit. It requires a firm grip... and..... this is where I get to the point..... I also wear a watch with a steel band that has a strong magnetic latch. In trying to push the plastic leg onto the Spark, it is quite possible to have one of motors  touch the magnetic clip while handling things and magnetize the motor housing. So, to prove a point, I had calibrated the compass yesterday and looking at the arrow in the app it pointed in the right direction. I had a test flight this morning and it was all good.
Now, I just put that watch band back on my wrist and fitted the clips... carefully watching on the sensor display to see what happens, and sure enough, it went right into the red while I was doing this. When I placed the Spark back on the ground, the interference bar returned to normal, but the number it displays was higher, about 150, before it was 12.. it does vary a a little bit just sitting there. However, going back into the normal screen, the pointer indication on the map was about 30 degrees away from where it was before!
So, I didn't fly, but I am pretty sure I would have got compass errors had I taken off, even though there was no warning on the screen about interference.
Long story... but maybe someone can analyze it better?

Hi, sir. Any interference around your spark? I would recommend you fly your spark in a place without interference. If you fly in a new place, please try to calibrate your drone's compass first. If it is not caused by environment, please provide the flight data and the black box for further check.

I've been paying more attention to the sensors too.
Just moving the remote above the spark shoots the compass into the orange red.
Perhaps that does disturb it more that temporarily.
Also the gyro can stray into orange, but recover or so it appears.