How anyone could lift a child out of an apartment and get clean away with so many tourists and locals wandering about is definitely a bit of a mystery. The other element to this is why even risk it?
Unless corroborated by other witness statements we know nothing about ... no-one saw Jes Wilkins and son on his perambulations around Luz. No-one saw Mr and Mrs Moyes returning from the restaurant where they dined, crossing the car park and entering the building. No-one mentions the woman Mrs Murat saw, indeed no-one mentions Mrs Murat.
Either people in Luz were singularly unobservant or it was really quiet with not too many people out and about at that time. Madeleine was very tired that night ... maybe other holidaymakers' children were too and most were indoors and dining at home given that we are told it was quite a chilly night.
Why would anyone risk kidnapping a child under any circumstances anyway? If Tannerman actually was the man who did, the risk paid off for him as when seen Jane assumed he was innocently carrying his own child home and the alarm wasn't raised till some time later.