Imo FW couldn't see her because he didn't step inside the room. The windowless room was pitch black and the body lay to the left of the door.Why did Fleet White not see JonBenet when he unlatched the door that morning?
When Fleet later watched John exclaim "Oh my God" before he had switched on the light and before he had stepped into the room, he knew instantly that something was VERY wrong.
I think JonBenet's body was already in the wine cellar before the police arrived. I can't imagine John going down there at 10 am and moving the body from another place into that room with the police aready milling about in the Ramsey home. This would have been far too risky imo.