WOW! To see one out in the open like that! It's easy to see why they are marked as they are since this one might be hard to spot against the ground unless moving. They are magnificent animals important to their ecosystems.
Years ago we were on an instructor-led geology hike in northern California. Matt was along, preteenish, and our leader stopped for a moment to point out a rock ledge at about eye level some six feet away. We looked, and umm, yeah? So rock ledge. He said, look closer but be careful and don't get too close. So we did and finally spotted a young rattler, maybe a foot long, tucked into the ledge, uncoiled (so less dangerous since they cannot strike from an uncoiled position), and eyeing us back. The simple fact is we would never have seen her if our leader had not spotted her. He was a dear man, a professional geologist born and raised in Wyoming. He had highly developed his skill at spotting rattlers since they are an accepted hazard of the profession. We stared at her and she stared back, probably raising her heart rate, but so long as she held absolutely still we would never have seen her. Humans are far more dangerous to them than they are to us.