Posted before about being on a flight from Madrid 16-17 years ago with couple of British early 20's students, they had been part of a Yacht crew that had left south coast and because weather/wind had been good they handed ended up in Northern Spain.
Weather turned meant they would have been stuck for least a week there so after being let into Spain they made their way to Madrid for a flight leaving skipper with the boat.
Their parents had booked the flight with airline "Go".
Airline called UK immigration who asked to speak to the teens, after handing phone back to checkin UK Immigration just said, "yup sound like they are our just send them on back and we sort at Stansted".
Youngsters a bit surprised but airline staff said happens all the time, in this case their parents had to bring passports to Stansted but lost / stolen passports is such a frequent occurence that airlines have process for this.