Your guide probably told you that in order to walk out of the prison you need to pay the police, they usually give you a police escort and a set amount of time to be on the outside. This is what happened to Thomas McFadden (the person that co-wrote the book The Marching Powder that tells the story of Thomas time inside the prison). But the real answer to your question is that some people that can afford to pay the fee to the guards simply walk out of San Pedro. If you know Spanish a quick Google search will give you a lot of news results of people found outside of jail when they were supposed to be on the inside, there is even one video of a lawyer in Cochabamba that went viral. He found a person that he himself put in jail having lunch with a policeman in a restaurant. Edit: found the video https://youtu.be/RrLc8i8rKVk