Lost iPhone and sharing passcode

I can see they might want proof it is yours, but I would not provide your passcode because that would open up the phone to the people in possession of it.

If they want proof, try having a message display for them telling them it is yours, or some agreed upon message.

Click on this link to read about "Use the Find My app to locate a missing device or item" --> Use Find My to locate your lost Apple device or AirTag - Apple Support

- "if you want to enter a custom message asking the finder of your missing device to contact you."

Read this support article about all the things you can do if a iPhone or iPad is lost or stolen -->

Find your lost iPhone or iPad - Apple Support

If it is 7 km from the airport how do you know you are speaking with airport officials and not thieves or somebody trying to get into your device?

Not getting the message could be due to no internet connection but doesn't it have cellular data activated? Also if you see it in Find My then it is connecting. If you are speaking with thieves they could be lying about seeing no message.

My inclination is to be neutral with the people at the other end in case they are airport officials, but not to trust them at all. There should be come way to resolve this without compromising your account and device security. If you look at the location in Find My, is that an airport building?