Tim Hardwick, MacRumors:

In iOS 15, Apple added separation alerts to the Find My app, which let you know when you leave an AirTag or other Apple device behind. Thanks to an October 2021 firmware update, AirPods Pro now also support separation alerts. This article explains more about how they work and how you can set them up.

Nice writeup, easy to follow. Key bit is setting up a Trusted Location, a place you can leave your AirPods Pro behind without a notification.