The natural reaction would be to run in the opposite direction of the opened window. So that would mean into either into the lounge, or into the other bedroom - I am not sure which. (Another possibility would be to run out onto the balcony, however I think that is less likely).
In one of the statements in the files it was suggested that holidaymakers were carelessly leaving doors unlocked enabling burglars to gain access easily. This suggests that there was no obvious sign that the accommodation had been entered and exited by the expediency of lifting the shutter and sliding the window.
I think they used keys to enter.
As you say empty properties were preferred suggesting surveillance, although that went wrong in Mrs Fenn’s case.
The independent witnesses who recorded instances of men watching 5a are too numerous to ignore.
I think that taken in conjunction with the phone calls at critical times, for example when the family picked up the children from the crèche, there is compelling reason to suggest the McCanns were under surveillance.
I also think that there was no intention to burgle 5a, I think Madeleine was targeted.