Why is my iphone 7 battery draining when powered OFF?

I'm telling you it's a new iphone 7. I work for a large municipality and have just been given this and it's new...brand new. The battery life is 100%. On two occasions I've powered it off because I don't use this phone number and both occasions I've tried to turn it on and it's dead/drained. Yes I know how to power off. Yes I also am somewhat techie so asked on here because I'm stumped.

I don't know why you're arguing with me. I mentioned I work for a large municipality so it could have been sitting in a store room for months. The battery life is 100% so unless you have anything helpful to recommend I suggest that you stop arguing with me and wasting my time.

It's a new phone...100%