Historically people put a note saying this bike belongs to "Name, address, phone, email" in a few places, such as inside the handlebar tubes or the frame's seat tube or bottom bracket, between tire/tube, etc. Some went so far as to put a "dummy" tag on an easily seen location (top tube) in the hope the thief would see that feel all is OK after finding it.
However, with the advent of the the airtag, I don't know if the tag would a) fit inside inside those places and b) if it does, if the tag's signal could get through.
I would experiment in places like under the seat (covered in black tape), under the brake hood, maybe wrapped under bar tape (street-side of bar so doesn't interfere with hands), etc. While touring, you could but inside a pannier I guess.
However, all that may not matter if you do not have unique identifying marks on the bike other than the serial number if the bike is not recovered immediately before the air tag is removed. I had a Co-Motion Americano with Rohlof (no air tag) stolen from our garage most likely by a homeless guy. When the police took the report, I was lucky to get an officer who rode bikes. He said if I had pictures or could describe very specific deep scratches, dents, etc. that would be hard to remove/hide otherwise it would become a he said/she said scenario and the "possessor" would win even though my specific bike (make/model/options/color) was pretty unique, especially for a state the size of Oklahoma. Fortunately, the bike did have a small dent on the top tube.
Months later, the bike was recovered. The thief ground off all serial numbers (bike, Rohloff, dyno hub, etc.) and spray painted the bike a different color but it was the dent that finally allowed me to get the bike back since no other identifying mark was available. If I did not have a dent, the police would not have returned it to me do to liability.
All that said, if you are touring in mostly rural USA/Canada, I would not be overly concerned. I have toured for 45+ years with probably more than 75k miles of loaded touring and have never had a bike stolen. Sure it could happen but most likely not.
I always carry a "opportunity theft deterrent" bike cable (a 10-foot 1/8" vinyl coated cable with a small suitcase lock long) and use it if in doubt. I figure a professional thief would get the bike regardless of the security measures so just try to prevent the opportunity theft by the person walking by. The local hardware store (not big box) can make one to length if you want. Lately, I bought a "dog tie out" cable with a thin cable and loops on Amazon so I didn't have to make one and have no issues with it.
Hope you have many great tours! John
