I have to echo much of what has already been said above. With the amount of panels that have been used for firearms target practice, be it the distant past or in recent history, and given the incidents of other types of vandalism to various well-known panels, even as recently as this last year (i.e. Wolfman), it's no wonder why those who have found their way and/or discovered lesser-known panels and archeological sites would keep it close to their chest. There's no guarantee that what's left of the undisturbed panels that are out there would remain that way if anyone ever made their whereabouts publicly known on the social interwebs. It's just the unfortunate fact of life that there are some out there who don't reverence such spots with the respect and awe that people like us have for them. I've seen some argue that if carving, painting, and/or writing on rock walls is vandalism now, it was vandalism then and that they have every right to express themselves and leave their mark as those in the distant past then. And to such folks, I always argue back that those in the distant past did not have paper, canvas, film, cameras, the Internet, and the other modern mediums we now have for documenting stories. That makes a big difference in my mind.
Anyway, some good tips on where to go about exploring have already been given above. In fact, I would dare say that just in the Bears Ears/Cedar Mesa region alone, almost every mile or two of every canyon that cuts through that area will offer up a ruin, archeological artifact, and/or art panel of some kind and if you're only able to make it out there for a short trip once or twice a year for the rest of your life, you'd only scratch the surface in all that can be seen and discovered down there.
There are plenty of well-known panels throughout the southwest and beyond that could keep one occupied with a trip or two out there every year. Many of the others whose whereabouts are lesser known can take some extensive scrambling and remote navigation. I've had the good fortune of finding the location of a few such spots after some extensive detective work myself using a combination of both online resources, library books, and other resources and then privately reached out to some others who I knew had been there asking them to confirm if I was right with my location and route findings. So I will agree that doing a lot of that detective work beforehand, in addition to simply stumbling into other spots unknown to me while out exploring has often proven to be far more rewarding than some of the sites I have visited where there location and route details have been served up on a silver platter for the whole word to have.
I think in addition to doing your own detective work and personal exploring, focusing your efforts in areas with geological characteristics as described further above in this thread; it also comes down to just building up relationships of trust, if only virtual, through a number of interactions with those who have gone before and then conveying reverence and respect for such sites through reports of the sites that you are able to find whether it be via information you find available online or elsewhere. I have found that once someone can truly sense you share the same passion and respect for such sites that they do, they are usually more likely to privately share some details, or at least give you some clues or confirm any detective work you've done yourself for the location of any particular spot that is of interest to you. They also have have to trust that if you find it, that you'll in turn keep its location close to your chest and not publicly reveal it or disclose any major clues to anyone else that doesn't surely hold these pristine archeological resources with the same high regard. And that's all a network of trust that has to happen organically.