Hi folks, something to think about...
Here's sommat I noticed the other day when my friend  was fruitlessly trying to do a range test with a 20mph gust headwind.
Sat on the rear of the open tailgate, the controller's direction on the Attitude indicator was way off, by about 90º counterclockwise.
Started moving away from the car and it turned back towards the direction of the ac, around 6-10ft was the distance needed to
resolve the issue.

My point is, the car was turned off, even the internal lights were off, keys out and still it gave off magnetic interference, so those
that take-off from the roof of the car, you need to be mindful of this, like we've said before, just because it hasn't happened
doesn't mean it never will Please see the video link in this post and watch what can happen, its not a car, just showing you what you
should expect to happen and be ready for it, I would also make sure there are no trees or structures for a good few hundred meters,
around the car, at least then you stand a chance of not crashing.

My point is, the car was turned off, even the internal lights were off, keys out and still it gave off magnetic interference
You seem surprised by this but it's completely normal.
Cars are made of steel (lots of it).
And compasses are attracted to steel.

The phone or tablet has a compass sensor.
When that's close to steel, the compass sensor gets deflected (just like a magnetic compass would be) and the indicated direction will be wrong.

Not surprised because i know the cause and effect, from experience, I was trying to highlight the point, I was more of a "I should have known", I just hadn't noticed it affect the phone before.

Hello there Bashy. Thank you for reaching out and for sharing these information and for sharing your experience with regards to this matter. Just a reminder that when taking off from metal surface, please take time to check you compass arrow is pointing on the right heading. In addition, the said DJI drone might have picked up magnetic interference from metal objects that is why the said DJI drone had that incident. Thank you and fly safe always.

Hi Bashy, I did a test about ferrous materials like a huge cannon, a car and rebars and what happens to the compass. Have a look at this https://forum.dji.com/forum.php?mod=viewthread&tid=247254