Lurking means exactly that. A stranger lurking suspiciously in a place where he had no business who moved away when seen. Had it not been suspicious it would not have been reported to the police.
But how does that constitute clear proof that the McCann's and an abductor were actively watching the McCann's?
Also if the McCann's were being watched then wouldn't the abductor know they checked every 30 minutes or so?
If so why would he attempt to get in whilst Gerry was there or just leaving?
If the statements are true he had a window of opportunity of around 1 minute 30 seconds to perform the following:
Enter the apartment
Sedate all three children - in the dark
Select Madeleine as the victim - in the dark
Open the (curtains) shutters and window - if he used the front door to enter
Pick Madeleine out of her bed - in the dark
Turn her round so that her head is now to his left, rather than to his right,
which is the way he would have approached her in the bed.
Exit the apartment, either through the opened window and shutters, or through the front door, which he must then close silently behind him.
So bearing in mind what he could could achieve in 1 minute 30 seconds why would he take the risk of entering the apartment right at the point of Gerry leaving and being on the street next to the apartment when he knew if he waited 5 or 10 minutes until after Gerry had left the scene he would still have had enough time to perform the abduction.
How would he know for example that Gerry would not go back into the apartment after his chat with Jez?
It's a huge risk and completely nonsensical.