According to a report from The Verge (via 9to5Google), they got in touch with the owner of the handset, Brian Green, who confirms that he got his replacement unit from AT&T. He told them that there was proof that his phone was a replacement unit as the battery status indicator was green, and the box the phone came in also had a black box marking to indicate that it was a new unit.
Green claims that his phone was not charging and he had actually turned it off at the request of the flight crew, but moments after putting it in his pocket, he noticed that the phone began smoking and he quickly threw it to the ground. Thankfully the plane had not taken off and the flight crew managed to evacuate passengers.
In a statement issued by Samsung, “Until we are able to retrieve the device, we cannot confirm that this incident involves the new Note7. We are working with the authorities and Southwest now to recover the device and confirm the cause. Once we have examined the device we will have more information to share.”
In the meantime the phone is said to be in the hands of the local fire department’s arson unit where it remains under investigation, so the exact cause of the phone catching on fire despite being powered down is still a mystery.