No Kate looked out of the window. The window could have been opened for a few reasons. Abductor could have come in through the window, abductor could have opened the window for a means of escape if caught in the apartment, abductor could have opened the window to pass Madeleine through to an accomplice.
IMO the abductor was disturbed by Gerry coming back to the apartment, one of the friends listened outside the children's window and went back to report all was well. IMO the abductor was watching the friend do his check and decided there wouldn't be another one for thirty minutes, so he went into the apartment and was disturbed by Gerry coming back. When Gerry left the apartment he opened the window incase there was another disturbance and he could make an hasty retreat.
Too many assumptions here imo.
You're assuming Kate looked out of the window, when only she really knows.
You're assuming there was not only an abductor, but also an abductors accomplice.
Worst still, you're assuming the gender of this imaginary abductor.
But other than that, I agree completely.