There's definitely lots of undocumented rock art throughout NV (and other parts of the SW). Valley of Fire has tons of petroglyphs, lithic scatter, pottery sherds, some pictographs, old habitat areas, etc. Most of it undocumented, as the park only highlights a very few well known locations (Mouse's Tank, Cabins, Atlatl Rock, etc.) to help alleviate some issues with vandalism & theft. The reality is, no one truly knows what the majority of rock art symbols mean. If you're interested in learning more, one of the most well-known books is "The Rocks Begin to Speak" by Lavan Martineau. It's even heavily used by several respected university anthropology departments. Nice find and happy exploring!