The reason the intruder went through the charade of putting the chair at the window and cutting the screen was because he knew the fact he had a key, knew the family didn't use the alarm, and knew he could use the key to walk in the front door would lead police to his accomplice and planner of the kidnapping, Ricci. Ricci knew the police would be coming to visit him anyway.
The police would know, as someone who had access to the house for an extended period, Ricci would have had opportunity, as Ed Smart has said, to have gotten a key. Who else would have tried to hide the fact they came in the front door with a key? Uncle Tom Smart? The Mormon bishop? No, it was a Ricci accomplice, to be sure, executing Ricci's plan and covering for Ricci.