You lost your passport in an airplane what should you do?

Call the airline and ask if one has been turned in. If that comes up negative, then you can apply for a replacement/duplicate from the government. No doubt, there will be a fee charged to replace this important document.

They sell these zippered pouches that one can wear around their neck underneath clothing ... used one with great success - also becomes a secure place to store identification and money. Being as it is invisible beneath your clothing, it's not a temptation to thieves.