Given the police were less than satisfied with the cooperation from the parents it seems only natural to me its standard procedure to look closer at people suspected of lieing or witholding information.

Assuming it's not the parents, the detective agency may be close in thinking an employee tipped somebody off. I can't quite see that though because of the narrow time window, but then again maybe not so narrow if the parents had established a routine. My money goes on an employee either as accomplice or killer.