Somehow I don't think they would have walked out on their own. Interesting you should mention that though since the McCanns locked their apartment during the day when it was empty but left it open in the evening with three toddlers inside. Some logic????
So the absence of the items I mentioned would be evidence of someone entering the apartment illegally and stealing them, correct? Of course there may be other explanations, even though these items can't walk - such as - you forgot you absent mindedly locked them in the safe before you left, your other half moved them to the bedroom and didn't tell you, you staged a robbery for insurance, etc. Nevertheless their absence is also evidence of a robbery, if indeed one has in fact taken place. One couldn't report it as a robbery without some evidence of it in the first place, therefore a child's disappearance is also evidence of an abduction, though abduction may not turn out to be the actual reason for the disappearance.