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.
A few days ago I found some academic papers studying/explaining how well various sources of cattle did in a desert environment (results- duh - indigenous cattle and their offspring did the best) .
The BLM Helicopter Cowboys apparently gave themselves 30 days to get the job done. They still would have likely missed a few.