If you read his statement to police he indicates that he sometimes forgets and leaves certain doors to his house unlocked, from time to time, accidently.
If he was anal, he wouldn't have left it unlocked, necessarily. The supposition that he called his son to have him check means he thought he might have GOOFED AGAIN and left it open.
The context of DW telling the police about supposedly leaving a side door open is to explain why he told them he thought Danielle may have gotten into his house. The excerpt I posted last night is just a snippet of what he said.