The device was found along a roadside in Oregon, according to game designer Sean Bates -- who posted on X about his shocking discovery, saying it had no scratches on it, and worked just fine.

He says the phone was still in airplane mode, and it unlocked without a passcode. Most telling, he says it opened to a baggage and flight confirmation for flight 1282 ... the flight that was forced to make an emergency return to Portland airport after the door blew off mid-flight on Friday.

In a follow-up video, Sean said he was skeptical when he stumbled upon the phone, which still had a piece of a charger wedged into it ... but called over a member of the National Transportation Safety Board search team when he realized what he had in his hands.

The plug used on the exit door has also been found, according to NTSB Chair Jennifer Homendy -- who said it was found in a schoolteacher's backyard in Portland after Friday's ordeal in the skies.

As we reported, a wild video from inside the plane showed the insanity folks had to face while on board -- with strong gusts of wind filling the cabin while the Alaska Airlines pilots had to fly back to Portland to land.

While the plane -- which was heading to Ontario, CA -- luckily landed with no injuries, there have been reports that pilots had filed reports about warning lights about the plane in the days leading up to Friday's incident ... which indicated a loss of cabin pressure.

The Boeing 737 MAX 9 was restricted from taking long-distance flights over the water as a result, and Homendy told CNN she was aware there were issues with the aircraft before the ordeal went down.