If GBC threw her iphone into bushes or a creek, around what he will describe later as her usual walking route, he probably accessed it remotely , later , to disable it and all of its contents. He would want to ensure that no one ever finds out the incriminating information it contains.
It's easy to wipe its contents and disable it from being located.
He knows he can do this just by logging into her iPhone app " find iPhone "
He is now very confident that no one will ever get information off it remotely,
But, just in case it is suspected and proven that he accessed this app and could do this, he sends a text to ABC saying specifically " app not working" because to know this, he has to access the app in the first place.
This text is so obvious. It's his excuse for accessing her iPhone remotely. To find her. Good cover.
I suspect they later told him it was traced to around the house to watch his reaction to this information and hopefully catch him if he checks it or tries to move it, if he did still have it hidden somewhere.