I found the exact same thing while cutting up my buck this year. About 4" of arrow shaft and a broad head that penetrated in through the back strap. There was no visible wound on the hide I saw or that I even noticed while skinning. I didn't see anything till butchering. The broadhead was actually poking through the tenderloin on the inside and I was lucky I didn't slice my hand open when blindly reaching in to gut my deer.
