What they never told us

This image is just as intriguing to me as the messages thrown into the sea in bottles. Messages from unknown senders, who made an attempt to have told us what they never told us.
 
 In the middle of the desert in Namibia:
 
 What could this sign say that I needed to know? What prohibition did it establish? What danger would it have wanted to warn me of?
 
 Valuing the mystery more than knowing the truth, I did not go to see what this message said, thrown into nothingness, and although I have my hypotheses, I will never be able to know what it said.