Could something like this work?
https://www.apple.com/uk/airtag/
Seems good apart from it appears it would notify the thief (assuming they have an iPhone) that they have an Airtag with them that isn't theirs - i.e. warn them that they are being covertly tracked..?
@Philipflop started
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Could something like this work?
https://www.apple.com/uk/airtag/