This happened to me last month.

Another passenger helpfully "found" my passport and handed it in to the lost property at Madrid Airport while I was doing a final preparations check to leave the lounge.

I ended up missing my flight, but was rebooked FOC by the Iberia desk in the lounge (IB ticket, IB metal only, and a very cheap fare - was placed on standby but all flights went out full that day) and was only out the cost of the Hampton by Hilton, Alcobenas, plus a return taxi ride. Could have been a lot worse.