Update: scratch the note below. See here for current status; the family was well known to social services and the police before this broke.
Parents in California have been arrested for a slew of crimes in California, and apparently they've been hiding under the aegis of homeschooling. Earlier cases have almost invariably fallen into the category of crimes where the perpetrators were already well known to authorities, but such evidence does not appear yet in this case. The closest I can see is that the couple is said to have had a number of addresses, and (like the Turpin case) those involved in their moving might have noticed something awry.
Of course, there is yet the proverb "hard cases make bad law", and one doesn't want to institute Jeremy Bentham's Panoptikon any more than it already is, but at this point, this case does appear to be one of those harder cases.