Anyway they could only see the top third of the balcony door. And they couldn't see any of the west end (the opening end) of the balcony door - look at photos - a bit of the building sticks out which blocks their view.
And not forgetting the cacophony of a busy restaraunt and the particular loud party, (aka Bridget Wilkins saying their laughter wafted right up to her apartment)any theory that they would have seen or heard Madeleine on the balcony is a non starter IMO as is her spotting her parents and shouting to them. I agree with the idea if she walked out she would have gone to them instead. VIs a vis the outer reception door and ablity to open it, doesnt really matter, someone will have probably entered or exited at some stage, or someone pass by and see her struggling to do so, maybe...maybe these queries should be addressed to KM who seems to think her child was able to go out and go find them despite also saying She is sure she wouldnt have gone out the apartment