I threw one in the trunk of my car (just in case - I ordered a 4-pack and I had a spare one), and every single time I drive somewhere it chirps loudly when I'm exiting my driveway, making its presence immediately obvious without any delays, and despite my phone being with me in the car.
I have them on tons of my devices, including my kids personal items that go to school, etc. and they never chirp, and I can and have found items that were misplaced in public locations (but not actually stolen).
Something is off for sure, but it's not like there are any user-configurable parts here. I literally just threw it in the trunk - and it's not like there's a right and a wrong way to do it. :-)
I guess it's because I don't have a garage and my car is parked in a carport, about ~100 feet/~30 meters away from my apartment, so it "normally" doesn't sense my phone "nearby". Then, I suspect, when I walk down and sit in the car (which takes just a minute or two) there is not enough time for it to reconnect with the phone and realize the owner is around. Because an AirTag on my wallet doesn't do this. But that's just my guess - I'm too lazy to pull an SDR and listen to the radio traffic to confirm.