The issue of which door is being used for the checks is being discussed in the recent YT videos on the McCann case. The nidus for the discussion was the recording made of Philomena McCann where she says Madeleine was taken from a "locked apartment" (Sky news interview I think).
"In his 10th May Statement Gerry agrees that the patio door was unlocked.
https://youtu.be/44Yy29eFx78?t=195 compare that to what Gerry states: "Despite what he said in his previous statements, he states now and with certainty, that he left with KATE by the rear door which he consequently closed but did not lock given that that is only possible from the inside. Referring to the front door, while he is certain that it was closed it is unlikely that it was locked as [because] they had left by the rear door." So without a doubt the apartment was unlocked.
So Gerry may have continued to state he entered by the front door as per his 4th May statement but then it would be so unlikely that the windows were open at the 9:05 PM check for the window and front door are side by side and would have been obvious to Gerry.
It is still possible for Gerry to enter by the front door even if the patio door is unlocked provided he had taken the key with him.
So if Gerry had the view an intruder was inside the apartment at his check there are only two entry points available.
1. the unlocked patio door (no key required)
2. The front door if the intruder had a key.