https://wccftech.com/ios-15-separation-alerts-informs-what-device-you-left-behind/

Apple is adding a new Separation Alerts feature in iOS 15, and if you are someone that has a propensity to forget things or items easily when leaving the home or other places, you will find this addition a godsend. The feature will be added to the Find My app, and it will alert you through your iPhone if you are leaving behind an AirTag or any other Apple device.

Separation Alerts will also work for Find My-enabled third-party devices, so it is not like you are limited to just Apple-branded devices only. For any item in the ‌Find My‌ app, you can tap on it and select the ‘Notify When Left Behind’ option. Once enabled, you will start receiving alerts if the items present in the Find My app start going out of range.