Set Breaker #23: Liar, liar

The Setup

Mr. A was found dead in his home by Mr. B.
Mr. B recounted his horrific find to Detective C.
“I was walking by, when I thought I would visit him.
I noticed his study light was on and I decided to peek in from the
outside to see if he was in there. As I wiped the frost off the window, I saw his lifeless body. So I kicked in the front door and called the police immediately.”
Detective C arrested Mr. B for the murder of Mr. A.
How did he know Mr. B was lying?

Take me to The Set Breaker