 the site was obviously not prepared  for wrecking, there were no permits posted, and the demo plan would  not have matched the configuration of the "wrong" building (OK, so  this is a conceit of the movie, we can suspend the disbelief for  this one.) Also, the front end loader would not have been stopped  for so long after tearing out the first chucks of the apartment.  Yes, after seeing an occupant, work would stop immediately, but  there was an overly long stop period before they see Harold Crick.  (This can also be the writers conceit, but it truly doesnt fit the  situation.)