If there are elk in the area, I'd first determine which species made the rub. Those look like elk rubs to me, though it is impossible to tell without seeing them in person.
Elk will typically rub alder from about waist high up to maybe shoulder height, depending on the size of the animal. Blacktail rub sometimes at ground level, but often knee high up to waist high. A buck averages somewhere around 40 inches tall at the back, so when they bend their head down to rub, it won't be a lot higher than that. A close inspection of the rub often reveals animal hair remaining in a rub that can remain for several months. Long and black: elk. Short and gray: deer.
As far as hunting rubs - some people just sit hidden but close by and wait it out (if they the bucks are actively rubbing). Some set up for rattling or grunting sessions believing the animal is relatively close by and you are an intruder into their territory. Many don't hunt rublines at all. (that's my take on it anyways).
