Strange battery drainage situation

My iPhone 7 Plus does not normally have a battery drainage issue. I will typically lose a few percentage points if I leave it on standby and laying on my dresser overnight. But a while back I lost almost 80% in one night and it repeated itself the second night. It charged and drained like normal during the day. The only thing I had done differently those two nights was I left my phone laying face down on top of a library book I was reading. It was a hardcover book with one of the shiny plastic covers they come with. After a few more nights of experimenting I confirmed it was indeed the placement of my phone on top of the book that caused the excessive drainage. Has anyone else encountered something like this or am I living in my own personal Twilight Zone?