Why is my iphone draining even after I turn it off?

My iphone was at 40% so I turned it off so I could keep the power at that. But when I turned my phone back on it was at 15%. What is causing that?

You might have some app's running. Turn them all off. Your phone is not off, even when you turn it off. That is why, if you don't want the law to be able to track when you are on the run. You have to remove the battery.