I had a gut negative reaction to this comment (the government comment is irrelevant).

But to play devil's advocate, can't someone buy any cheap old iPhone (or any smartphone), activate it with a cheap prepaid plan, and use the phone itself as a tracking device instead of an airtag?

I guess the only thing special about the airtag is it's (slightly) cheaper than a cheap smartphone, longer battery life, and explicitly marketed for the purpose of tracking things. Even if a cheap prepaid smartphone can accomplish the same thing while going completed undetected.