If a thief has an iPhone, they will be notified in 3 hours that they are being followed by an Airtag and can activate the buzzer to help them locate it. Here is a solution that might help keep it hidden longer if you are confident enough to dismantle and reassemble your Airtag.  https://www.youtube.com/watch?v=gSRc1DScOY8&list=WL&index=13