Looked at in the components of that scene, in sequence:

Anthony's drawing was neatly laid on Michael's pillow, for Michael to see. Anthony probably put it there.
The light was on Michael's side of the bed. It had to be on for Michael to see it when he came into the bedroom. Anthony may have put the lamp on for Michael to see it.
He probably went to bed before his parents, so the drawing was in place and the light was on before Kay went to bed.
Kay was asleep, facing away from the lamp, when Michael came in.
Michael, after viewing the drawing, woke her and asked if she'd seen it. She said, "Uh-huh." She may have left the light on so that Michael could see the drawing. Then she asked, "Why are the drapes open?", meaning that the drapes had to have been closed when she got into bed.
The drapes had to be open, and the light on, for the killers to draw a bead on Michael when he came to bed. SO...
The traitor had to have entered the bedroom after Kay fell asleep in order to open the drapes.

The finger points at Fredo. In the later boathouse scene, he admitted that he betrayed Michael by giving Johnny Ola "information" about the "big deal" Michael and Roth were negotiating. That's BS: Fredo wasn't in on the deal--what "information" could he have given Ola? His only value would have been to open the drapes. But, that would have been incredibly risky--how would he have known that Kay was asleep? And, how could he be sure he wouldn't wake her?
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That's the long answer--and it's not very logical.

Short answer: Directorial license.