I have been travelling around with someone who has an AirTag on them, and it triggered Apple's "AirTag Found Moving With You" notification.
It was neat to see it work, and properly (eg: this AirTag wasn't tied to my account, and it was following me). It is the first time I've actually seen the notification (which did not concern me as I knew who was carrying the AirTag AND tracking it).
Previous writing on AirTags:
https://www.hotelexistence.ca/exploring-bluetooth-trackers-at-geekweek-7-5/
https://www.hotelexistence.ca/further-thoughts-on-stealth-airtags/