Well what i've noticed from our runners is even though we let em out to range, they still don't go far from their coop. We've had em about two weeks now, and they have just started to free range with our chickens. Ours will get in the taller weeds, behind plywood, under our pool cover, under our truck, and they LOVE to hide in the chicken coops roostin box where all our chickens spend their nights! Hope this helps, cuz we too have 4 acres! We were worried about letting ours free range that much space.
Our new duckling who is a week old we let outside with supervision, and she doesn't go far from us or the back door, but she does like to hide! In the pond rocks, bushes, under our outdoor chairs, in the dog house.!
Hope you find your babies!