how do i report a lost airtag on someone else’s apple id that i’ve never seen in my life

The owner can place the AirTag “Lost Mode” using FindMy and post an online message w/ return instructions. (very similar to using FindMy to place a message on the screen of a misplaced iPhone)

This message - if added - is accessible from an Apple website by scanning the NFC signal of the airtag.

Also provided is a facility to contact to owner.

https://m.youtube.com/watch?v=tOWk1TBq-Ek

You’re certainly welcome.

Hope “the system” works and that the owner recovers his/her device.