Marco, it's possible that one of Ola's men opened the drapes, but it isn't likely. I believe security was tight at the compound despite the hundreds of people present. Note how Rocco keeps scanning the crowd. And, as soon as Johnny and his men arrived, Michael dispatched Rocco to get them some food. Maybe it was hospitality, but I bet Michael wanted to have them under surveillance, out of general cautiousness.
The reason that I (and perhaps many here) believe Fredo opened the drapes as part of his betrayal of Michael is because, as a member of the family, he could come and go in the house without being challenged by the guards that Rocco undoubtedly posted around the house to keep curiosity-seers (and pilferers) among the guests from entering.