I think from what ive seen a terrier marking on a lead is a north/south divide thing. Up here you can check a lot of empty holes before you mark something whereas in most of the southern counties there's plenty of game so you can be pretty sure most places will hold and the terrier can be left off to find etc, but if you did that up north you'd be wasting a lot of time.
And every dog marks places differently. Some will whine, some will dig at the entrance etc etc, its up to the owner to notice the change in the dog and figure when the dog is marking a place as occupied.