Airtag Anti-stalking Sound

I have an Airtag on my car keys to help find them if they are misplaced and one hidden in my car on the off chance it gets stolen (hopefully to be able to pass on tracking info to the police). When I drive the Airtag on my keys is in very close proximity to the Airtag hidden in my car; both are in very close to my iPhone, which is mounted to the dash.

When someone else with an iPhone is in the car, the hidden Airtag makes the Anti-stalking sound, which is annoying as I want it to go unnoticed (the whole point of being "hidden").

I understand that Apple have taken steps to limit their liability around stalking and in doing so, removed one of the handy features of the Airtag (when used for legitimate purposes) but my question is; why does the hidden Airtag in my car make the sound but the one on my keys, which is essentially subject to identical conditions, does not? The only difference between the two is that during set-up one was set-up with the predefined label "keys" the other was set-up with the label "other" which I manually entered as "car".