iPhone 6 battery draining in hours

Hi,

I've got an iPhone 6 that is draining it's battery at around 8%-10% per hour without any use and in airplane mode. I've performed a full restore and at the moment the phone is set up as a new phone, is on airplane mode and is running in low power mode. Even so it's still draining the battery at 8%-10% per hour. I took the phone to an Apple store and they ran a battery test...it passed this test. They said a battery replacement would fix the issue but I don't understand how it can pass the battery test yet be draining in around 10 hours from 100% when I'm not even using the phone. I've noticed the phone is sometimes hot on the back also.

As mentioned the phone is set up as new so no apps are running in the background.

I read so many stories and forum posts about battery issues and I can't find anything that replicates this issue. Has anyone seen this before and know what I could do to fix this?

Thanks for the help.