I don't think Fredo might have opened the drapes and i think Michael knew that because if Fredo was just completely stupid and really thought it was a hit then Michael would have probably kept him alive, but Michael still had him wacked, I think Rocco killed the Tahoe assassins and that is why he went on the suicide mission.

"Well you just do what I tell you to do! Goddamn it! If I had a wartime consiglieri -- a
Sicilian -- I wouldn't be in this shape! Pop had Genco -- look what I got." - Sonny Corleone