Amazing how well animals can hide in plain sight. The best example I personally saw happened some years ago. I was driving alongside a mountain stream in the north GA mountains when I rounded a curve and spotted a small 4 point buck standing in the middle of the stream.
He laid down in the water with only his nose, eyes, and small rack above water. The water was shallow, probably not more than a foot deep, but with the water running over his body it looked like a rock just under the surface. Had I not seen him before he lay down, he would have been just as hard to see as that mountain lion.
I stopped my truck to watch. After 10-15 seconds I guess he figured out that he'd been busted. He stood up, shook the water off and casually walked to the other side of the stream and into the woods. And it was cold. In January with a dusting of snow on the ground that day.