Why does Belarius think Imogen is dead in Cymbeline?

The wicked British queen gives Imogen's would-be killer a phial of poison, telling him it was a cordial for Imogen should she get sick. The phial actually contains a potion which leaves the person drinking it in a death-like state for a few hours. After Imogen joins Belarius and his two boys, she feels ill and takes the potion. Belarius and her boys take the seemingly dead girl to the woods.