The cops still need a warrant to go into a house. You know, that pesky 4th amendment thing gets in the way from cops just making their way into to wherever an app tells them they should go.
The phone in the house does not prove that the thief is in there, or which thief is the thief that stole the iPhone; thus a warrant is needed to enter the house.
Your argument is a good and strong one but is it enough to convince a judge he should allow cops to force their way into a house?
Seems like a good chance a judge would give a warrant, but can't a cop just knock and announce to whoever responds that the stolen property is in the house, requesting its return and some information about where they got it? That doesn't require a warrant.
PS What's the difference between this scenario and a cop responding to a Lojack signal after a car theft?
