AirTag notification confusion - what actually happens if......

I am so confused about AirTags and notifications.

I recently lost a car key, so I thought it would be good to buy an AirTag to put on the keyring. I also wanted to place one in the actual car.

The car is shared, so it follows the AirTag on the keyring and in the car will be separated from my iPhone if my partner uses the car.

What will happen if my partner takes the car on a trip for a week. If it helps, my partner is part of iCloud Family Sharing. Will either/both start making noises after a certain amount of time?

TIA