Who is saying it was the abductor who was watching 5a? It could have been someone passing the info on.
In the UK the police ask the public if they have noticed anyone hanging around before a crime was committed, these people usually come forward without the police having to contact them. Though sometimes it is the witnesses who describe anyone they see is helpful in finding these people.
In the Claudia Lawrence case for example, witnesses have come forward to say who they saw leading up to the disappearance of Claudia.
The way you mock and ridicule what these witnesses have seen really shows you haven't a clue about how the police operate.
Just sheer prejudice, Lace.
Witnesses reported seeing people in the vicinity of the McCann apartment whose behaviour caught their attention; when Madeleine disappeared, they reported what they had seen to the police as was their duty, and gave descriptions of the persons seen ~ job done.
Procedure then would have been to trace and eliminate these people from the inquiry ... as you say, that is how it works.
That DCI Redwood was putting out appeals to trace the men seen by witnesses proves that the initial investigation neither bothered to look for them, nor did they trace them ... and that is an inexcusable disgrace.