The agency already stated that they had no reason to believe Marcus was in danger. Why wouldn't they take the foster parents word for it? They were seemingly good people. The social worker could have, and would have called police and forced entry into the home if they thought otherwise. This was all mentioned in a previous article within the thread.
The bottom line is, no ammount of law, rules, tracking devices, programs, alert systems, background checks, etc can stop something like this from happening. It can't be predicted. There are plenty of bad people out there who have always maintained under the radar.