I remember reading that only about 90% of Bundy’s cattle were branded (given that no one else’s cattle were mixed in and this year’s offspring were excepted - a reasonable assumption apparently).
I’ve talked with ranchers who have rounded up three year old cattle without a brand on their own property. Somehow they slipped past the roundup crews. In other cases it wasn’t worth the effort to go and chase down some young bull that was spooked by peoples voices. Just let him go about his business and when he gets old enough or the feed gets poor enough, he’ll come down out of the hills.