I thought about it, but how would this work exactly? I passed the airport on a highway, so I didn't connect to any wifi. Maybe my phone probed some access point, but still this would require the said point to send my phones identity to FR24 servers. Or maybe it was my cell provider? I pinged a cell base station nearby the airport, and this fact was registered and sent to a 3rd party, and FR24 has a subscription with that 3rd party monitoring users? But that'd be surveillance on another level. Thus my first guess is something happened on my phone, FR24 somehow came up with a solution to go around various privacy restrictions of iOS.