I had the same issue for months and this is what I tried and it helped stopping the drain issue. After these steps I left the AirPods unused in their case overnight and did not see a single % drain. Left it at 100% both case and AirPods and found it next morning at 100% both case and AirPods. Prior to the reset I would see a 5% drop after 1-2 hours when left in unused state.
Steps:
1) Turn off Find My Network for the device under Bluetooth Settings
2) Click on Forget device to disconnect the AirPods from any and all devices currently connected to.
3) Turn off Bluetooth on all devices that the AirPods were being connected to to avoid the AirPods from communicating back
4) Leave the AirPods to drain out naturally. (Check for indicator light on the case - If it doesn't lit up it means its being discharged and you are ready to proceed to the next step)
5) Charge the AirPods back to 100% once again ( Green light indicator on case means fully charged)
6) MOST IMPORTANT STEP - Hard reset your AirPods to factory settings- Here is how
A. Place both AirPods into the charging case.
B. Open lid of AirPods case
C. Leave the case open. Locate the Setup button on the back of your AirPods case.
D. Press and hold the Setup button for 15 seconds. The status light should change from flashing amber to white.
E. Keep holding until the status light changes from amber to white
F. Close the lid. Your AirPods are fully reset.
7) Proceed to setup your AirPods with your device once again. Turn off Find My Network under AirPods Bluetooth Settings and leave the AirPods unused for a period of time. I would recommend 24 hours. You can check battery charge through these 24 hours regularly but do not use the AirPods so as to not skew the results. Once you see and improvement and confident the issue has resolved you can go ahead and turn on find my network under AirPods bluetooth settings (or leave it off if you wish) and start using the AirPods as required.
Hope these steps help !