When I lived in eastern Montana rustling was a big problem.
There were a few guys that everyone knew was a thief but not much could be done until they were caught. This is big country, and as the story RE rustling in the ION area, these guys were born and raised there and knew the country really well.
Sheep thefts were the big issue then for a number of reasons. Paint brands were easy to clip out and groups of sheep could be easily handled by a guy with a dog or 2. A good dog can load a group of sheep in a trailer parked alongside a tight fence pretty easily.
Also similar to the Oregon rustling story, the history of that area was rife with stories of guys who got their stake stealing mostly horses. Cowboys used to ride north into Canada and gather horses, drive them south to Miles City and sell them. Then the enterprising buggers would steal horses from down in the Yellowstone Valley and drive them back up north to sell them there. Crossing through their home area, but like a coyote, didn't do much thieving in their own back yard.
