He should have guessed that a rat's instinct would be to run. And what is Mogget's instinct if not to chase? He pursues the boy to the edge of the property and there he stops, watching the shifter's retreating figure. It's enough to chase him off the site. And a few seconds later, when the security guards come, he tells them where to look. With any luck, they'll find him. And if not, well, they've given him a scare. Perhaps he'll realise that it would be unwise to return.