Hi, and welcome to the Apple (user-to-user) Community.
Re: "AirPods Pro 2 case losing power when not in use ...
... goes from 90-100% to < 20% in 5-7 days"
Many battery charged items do seem to lose power when not in use.
Sorry I do not know whether this loss of power is excessive., or not.
(Many AirPod users would keep the AirPods in the case when not in use.)
Apple support has articles online that may be helpful:
Eg: Wondering if you have tried resetting the AirPods Pro to see if that helps at all:
How to reset your AirPods and AirPods Pro - Apple Support
Battery life depends on:
device settings, environment, usage, and many other factors.
Charge your AirPods and learn about battery life - Apple Support
More info: Charge AirPods (all generations) or AirPods Pro (all generations) – Apple Support
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You might want to check the charging case is actually charging correctly.
If your AirPods won’t charge - Apple Support
You could likely get the AirPods and case checked out by the experts if you want:
If you choose to visit an Apple Store: a Genius Bar reservation is recommended:
Genius Bar Reservation and Apple Support Options - Apple
Resources:
AirPods Repair & Service - Apple Support
AirPods User Guide – Apple Support
More support . info. if needed:
AirPods Support Website:
Find all the topics, resources, and contact options you need for your AirPods....
AirPods Support - Official Apple Support
All the best :-)