Another thing that is confusing, to say the least, is starting up / taking off from metal objects.
One would expect the drone to throw either a “recalibrate” message, or just refuse (which can happen).
This came up in a conversation over a year ago, so I tested taking off from a very thick manhole cover.
There was no issue. 
My posts that relate to this are here and seven posts below - and the general conversation thereabouts.