iPhone battery drain when device is off

Hi

After upgrading to the new iPhone X, I retained my older iPhone 6s Plus for backup use. I reset the phone and am on the factory settings. Even on this bare minimum setting, my battery is draining when the device is powered off.

I did a full charge & powered off my iPhone before going to bed. The next morning when I turned on the phone, the battery dropped from 100% to 62%.

Does anyone know why & how does an iPhone battery drain even when powered off?

Thank you.

Peter