Well, we know Sieg was in the basement in 2006 (unless the video is fake). He described the picture, so had either seen it or been told about it.
We don't know if he was there at any other point up to ?2010 when they had the builders in and chucked out the stuff to the charity shop.
We don't know when the drawing was done, but we do know the scarf was there no later than 2010.
If either/both scarf and drawing were put in basement other than on disappearance night, then by whom (Mme Clos? Builders?)
Though I do think anything other than on disappearance night is too complicated.