Oli, the question is, what do we assume?
Why do we assume the drapes were not thin?
Isn't any assumption that Fredo, Tom, Pentangeli or whoever could just walk into Kay's bedroom much more far fetched? So the only adult person who could walk into the bedroom after Kay was Michael.
The light was on, so the assassins could have seen probably more than just a fuzzy shadow on the drapes.
Assassins don't assume anything? The assassins in GF2 assumed they'd definitely kill Michael (according to our mainstream theory with the drapes open) before he could dive for cover. And they probably assumed they'd get out of this affair alive.

Oh, and sorry for my English in my last posts. I'm a little sick in bed, which makes it hard for me to write in English. Having fired an Ak47 god knows how many times, I know the difference between an assault rifle and a machine gun.