I have visited several of the classic and many of the more hidden sites in the past year and I'm seeing many more examples of human interference: cairns built near the motifs at Weetwood, turf removed at Millstone Burn, several stones brushed and roughly cleaned, flat stones hiding the motifs on the main panel at Midstead along with cleaned stone and a brush!
Even as an avid photographer, for me this is not on. If panels get temporarily buried maybe that's a good thing for their long term conservation. I go to visit mainly for the atmosphere of the site. Of course I like to look at the stones too in different light and in different moods. Perhaps the publicity rock art has been getting and now the accuracy of their location (at least in this area) is not so good. Most of the stones have been well recorded already. If it's not visble, leave it alone.