They weren't marketed but it was implied. If you forgot, they had to patch them a while after release due to stalking concerns so that "day one" point is moot.
> To discourage what it calls “unwanted tracking,” Apple built technology into AirTags to warn potential victims, including audible alarms and messages about suspicious AirTags that pop up on iPhones. To put Apple’s personal security protections to the test, my colleague Jonathan Baran paired an AirTag with his iPhone, slipped his tag in my backpack (with my permission), and then tracked me for a week from across San Francisco Bay.
> I got multiple alerts: from the hidden AirTag and on my iPhone.
