Edit: In bold**By Penny W.**I have thought out the whole point of Dumbledore showing Harry where the first horcrux was. First of all, Dumbledore seemed to know what to expect considering the orders he gave Harry before they reached the cave. Second, the mysterious RAB doesn't make sense. I can see Voldemort telling his Death Eaters about the horcruxes, but not their location. The only people who really knew of the cave incident were the lady from the orphanage, Dumbledore, and now Harry. Dumbledore didn't seem to know where the other horcruxes were located.
Theory no. 1- Dumbledore went to the cave before Harry, retrieved the horcrux, destroyed it, and had Snape make the same potion. Then he took Harry back, as a lesson to show him what he would be up against when he goes looking for the remaining horcruxes. Dumbledore and Snape had planned Dumbledore's death once they figured out what Draco was set out to do, and planned his death around the cave visit, or when it was time for Draco to carry out his plan. In turn, saving Snape's life, and the Malfoy's, although I don't think Lucius has much hope. Although now, Voldemort will be a little afraid of Snape, because he was able to do what
Voldemort could not. So Snape will have to be a little more cautious and watch his back.
Theory no. 2- When Voldemort learned that Lucius had allowed one of his horcruxes to fall into the wrong hands,and be destroyed, he panicked, searched out his own horcruxes and left the phony ones in their places. Voldemort is so afraid of death, that he would go to these lengths to ensure that his horcruxes are still intact. Dumbledore beat him to the ring, but I think he might have the rest. Voldemort knew that RAB was dead already, so he left the message to fool whoever went looking for them into continuing their quest in vain. Giving Voldemort time to carry out certain plans.
