How do you know?! Anyone who breaks into a house to steal a child is taking a big risk, even if they are watching the house to avoid being caught.
It's contradictory, you are not really trying to thrill seek if you plan something to avoid being caught.
The police analyse crime types, they are aware of organised vs. chaotic crimes where a child might be taken by a predator on a whim. They will know how these cases will differ in terms of what evidence
should be there. It's goes along with logic and evidence to try and work out the possibilities. An organised criminal will plan not to be caught, a disorganised criminal would not and may panic but would be likely to leave evidence as not everything was planned.
How a disorganised 'whim' child snatcher would hide a body or child so well is anyone's guess. Someone who planned would be more likely to get away with it.
One of the reasons to do a reconstruction is to work out what kind of criminal you are dealing with in terms of organisation and pre-planning.