Shape, shine, shadow, sillhouette, seizmic, absence of the normal, presence of the abnormal, all that stuff.
If your open field has hedge boundaries, why not just move slowly round them as the warrens may be in the hedges?
I have loads of places that I shoot at like that, just open fields, I just stalk round the edges, walk very slow, stop when I see a pair of ears, crouch and sight up in one movement, then decide if its a shot, or I need to get closer.
As Simon says, 'wind in yer face bud' if at all possible.
Its always that 'sentry' rabbit that needs extra special attention
Hav