It doesn't seem to matter that the phone is powered down, as shown by this incident on a South West airlines flight where the Note 7 caught fire after it was powered down.

Here's an excerpt from an interview by The Verge on this incident [1]:

"Green said that he had powered down the phone as requested by the flight crew and put it in his pocket when it began smoking."

[1]: http://www.theverge.com/2016/10/5/13175000/samsung-galaxy-no...