Has been found over 200 times in more than 200 different places !
But to answer to your question , nobody couldn't found it because they didn't follow the clues Waltz has given . To say how it's not there because someone else has searched the region before , IMO is a big mistake . Like Waltz said , you can pass beside the mine without be able to recognize the site . So ,someone has to know to exact spot to find the mine .
When Not Peralta ( ex Tnet member ) wrote about how he had found the mine in the knob's cave , he wrote :
" The area I talk about was searched by the best for a long time, some claiming one thing then another. The location was very tricky,but,findable. I have looked at its location many times on google. "