It is possible to conclude from the various reports that Fleet White couldn't see the body because it was too dark and he couldn't find the light. This would suggest that he didn't really have much of a look into the so-called "winecellar". He planced in. Too dark. Saw nothing. Moved on.
However, it is a fact that FW was taken aback when JR found the body in that room, so taken aback, in fact, that he spoke to the police (Arndt). He told her that the body was not in the room earlier. This is why he was gobsmacked that JR waltzed into the winecellar and located the body (even before turning on the lights). For him to have reported to Arndt (incriminating his friend JR) he must have been *sure* that there was no body in there earlier. Might he have been mistaken? Perhaps. But he evidently didn't think he was. He seems to have been pretty certain that there was no body there earlier. This is why alarm bells started ringing for him and he felt compelled to mention it to Arndt.
So we can speculate about how much Fleet White could see in the dark room but his behaviour tells us that in his own mind he knows there was no body there earlier. He wouldn't have spoken to Arndt if he wasn't sure.
The "winecellar", by the way, was never used to store wine. What is important to realise is that it was a place used to store Christmas paraphenalia (amongst other stuff). And the last people in there prior to that day was the husband of the Ramsay housekeeper and one of his friends who were doing odd jobs for the Ramsay's helping set up for Christmas. This, imo, is why the body was put in that room. Just as the ransom note was put at the base of the stairs. In both cases those placements are designed to direct attention to the housekeeper and her "two gentlemen" (who don't particularly like "fat cats" like JR). IMO the Ramsay's were trying to frame the housekleeper and co.
At least in the first instance, they were. When that didn't take hold they started all sorts of stories including the intruder story.
One of the reasons Arndt was left alone in the house (with the parade of guests and visitors) was that the Ramsay's had already put the cops onto the housekeeper LHP and they were off to speak with her. Patsy at one point asked Arndt what was going on and Arndt assured her that cops were off to find LHP. The housekeeper was the prime suspect at first.
So why was the body in the winecellar? Because LHP's husband (and friend) were the last people in there. The location was designed to point to LHP. As too was the location of the ransom note. When Patsy was shown the ransom note she says "Gee, it looks like Linda's writing..."