police have said they believe william tyrrell may be alive after six months .
they revealed this information has changed the focus of the investigation .
police have issued a strong warning for anyone found connected .
the parents of the toddler released a heartfelt plea for his safe return .
his mother spoke of the moment she realised he was missing .
she said she searched the house in circles telling him to yell out .
his mother said she knew he was missing when she could n't hear him .
the toddler vanished from his home in kendall , nsw in september 2014 .