For the umpteenth time it depends on the when and the how.... If it was 8:20 then no it's not plausible etc etc etc.
If you seriously believe there was an abduction then you need to tell us when and how. Taking a child that has been left in an unlocked apartment is of course possible. It's knowing the arguments for the when and the how that lets us then establish if such a scenario is plausible or logical.
I will tell you the when and how of what I think happened.
The apartment was being watched. The abductor knew that the checks were made every half hour or so.
The McCann's left for dinner and arrived at the Tapas Bar at eight thirty.
Matt did a listening at the window, reported all was quiet when he arrived at the Tapas Bar.
However Gerry decided to do a check anyway, he left just after nine o'clock.
When the abductor saw Matt do his check, he thought he had thirty minutes in which to abduct Madeleine. He enters through the front door having a key. However, he is alerted by Gerry entering in through the patio doors. He either hides or goes back out through the front door. When Gerry leaves, he opens the window, incase someone else arrives at the apartment. He either passes Madeleine through the window to an accomplice or he leaves through the front door.
When later Matt does his check, the room seems lighter, that is because the shutter is open but the curtains hides the fact. When Kate enters the room, the wind blows the curtains open to reveal the shutters are open and window open. IMO