I have a horse who does that, he lies flat out in the field with his mouth open looking like a corpse. He only gets up when you get close. But my friend has a horse who you actually have to poke and shout at in order to rouse her when flat out snoring in the field, which is worse!
The lambs doing it is bad cos they are so little so it's quite feasible they could be dead, they look too little to be as tough as they turn out!