At this point for you, it doesn’t matter is it’s connected or not to your devices. Your A1 chip is already stuck in an infinity loop load triggered by the firmware update to 3C16 (lots of people had the same behavior when their aripods are new and they update). You are going to keep experimenting the battery drain weather you use them or not (even if they are inside the case) at an average rate of 4-6% per hour (8 hours (overnight) = 40%) because the headphones are stuck, frozen or however you want to call them.
SOLUTION
The solution is the hard reset as I just explained, cause that would reboot the airpods.
After that you’ll be totally fine.
After that most people have a 10% battery drain during the FIRST 24 hours of not use. Except when they left the AirPods fully charged at 100% (they retain more battery when they are at 100%).
After the first 24 hours of continuously not use , we have not drain rate cause they AirPod get into an ultra low power mode after the 18 hours continuously not use (And people use them back before we achieve this time window). And by no use i mean inside the case. If you leave them outside the case, it takes 72 hours to get that ultra low power mode. And you have to consider that you CANT touch them during this 72 hours , cause the headphones have an accelerometer, sensitive to movements, so any touch should wake them up, and the counting starts from 0 again.
if you want to minimize the drain during the first 18 hours, you need to turn the Bluetooth off from all your devices, SPECIALLY desktop Macs and APPLE TV
Finally, if you want to retain all battery over night, perform a hard reset before going to bed and put them inside the case, so they are fully disconnected (This is the actual way to turn the headphones off).
Next time in the morning pair them back with you iphone.
The problem can happen again in the future, this time triggered by the automatic switching feature presumably, but you can still solve it temporarily with the hard reset until the bug fix comes out.
hope this help you