Why would anyone watch an apartment over several days? To establish the comings and goings of the inhabitants and decide on the best time to strike
What could they find out? See above
Did the McCanns and their friends evening holiday activities fall into something of a routine or were they all over the place every night, sometimes dragging the kids out, sometimes not?
to see if it was worth breaking into? oh and but well, they would have discovered that there were crying children in that room ( having been watching for several days/nights.) which would be a risk not worth taking, also they would know one of the doors was left unlocked... Do'h and the best time to strike would have been right after the McCanns left the apartment fr their free drinks and meal as' they abducting burglars' would have been aware that the parents would be 'coming and going all night' Double Do'h.
Because, well ,you see the thing is :as they claimed to be coming and going all night checking the children no one actually physically checked the children- so Maddie could have been 'abducted' 'stolen' 'kidknapped'just minutes after her parents left...