blunomore
Honorary Master
What could he have possibly observed in the flat 7 years after the fact?
It may not be so much that he saw specific evidence in the flat, but more likely he was informed about where the victim's body was found, where the murderer likely entered, where the blood spatter was (which shows from what side she was attacked), etc etc.
These are things that make a difference in the analysis of a detective, I would suppose.
I do wonder if the flat has been standing empty all this time or whether it is being occupied again, in which case the new owners had to give permission for his visit.