No, I didn't make any transactions at the time while I was waiting at the airport.
In that case, it's not clipboard malware, though it's weird that the hacker didn't touch anything but Bitcoin - unless the rest of the coins you had are not worth the effort. The only logical thing is that your seed is compromised.
I am also going to meet the airport manager if possible to explain the situation. If their wifi is not safe for public use, then they should take immediate action.
Are you sure you have connected to the official wi-fi from the airport? Such public places are ideal for what is called an
evil twin attack, and if you were connected to such a fake network and logged in to your e-mail or any other service, all your data fell into the hands of hackers.