Hearing a Mysterious Chime from Your AirPods Pro Case? It's a Feature

Well that explains the faint noise I've been hearing at night lately just after putting my AirPods away.

Speaking of the sounds you're meant to hear, I'm familiar with the low battery ones (both the AirPods themselves and the case if you put the AirPods in it when the case is low on charge) as well as the chime the case makes when it's put on charge. But every so often the case plays one when I open the case. Any ideas what that one's meant to be?

Could this be some kind of calibration they have to do to keep mic/speaker in spec for qualifying as hearing aids?