Alert when FOB Left in Car

On Dec 8 a Tesla transporting a rare 310-year-old Amati violin ($1M) was stolen from art dealer's Los Feliz home.  The dealer claimed that the fob fell out of his pocket and remained in the vehicle.  Most likely the thief didn't realize the prize he had corralled.

I'll vote up for him.

I'd like the car to sound the horn with one long or three short beeps if I leave my phone in my Model 3. I've done this twice now and the only reason I noticed was that my smart phone vibrated when it was out of range of my phone.

On my old 2017 Hyundai, if you left the key in the vehicle, it would beep from under the hood.

 

Reasonable.  But, how might the sound be detectable outside the vehicle?