Estrela Mountain Dogs are high-energy animals with a constant need to walk, run, and play. If your Estrela doesn't get enough, he will likely try to jump the fence and run off on his own.
Just due to the fact that your dog may not feel he is getting the attention he deserves, he may bolt on you when least expected. Other reasons may include the dog being abused or frequently yelled at. Even if there is a lot of yelling or other loud noises in your home not directed at your dog, he might want to get away because his hearing is much more sensitive than yours. Also no dog enjoys being mauled or harassed too much by children or adults.
An Estrela that knows he's loved and well cared for doesn't run off. That should tell you something. That is not to say that you should coddle your pet or let him get away with whatever he wants. Nothing is better than some good, solid training to keep him from disrupting the entire household.
However, when it comes to your Estrela Mountain Dog running away, there are some easier solutions. You need to determine why the dog took off on you in the first place. Was he trying to get away from you or did he have a desire to find friends or greener pastures? Once you know the problem, it is easier to find the right solution.
Aside from taking off due to the need for more exercise, your Estrela might want to seek out canine friends to spend time with or other people from whom to get the attention he is missing. If loud noises seem to be the reason, do what you can to minimize the problem. If it is noisy children that upset the dog, perhaps you can take him out for a walk while the children play. If that isn't possible, there are professional dog walkers who can fill in for you.
Your dog might also be foraging for food if he doesn't get enough at home. In some cases, he might simply be bored and take off to find some excitement. Whatever the reason, never punish your Estrela for this behavior. He will likely resent you even more and run off whenever he gets a chance.
The ultimate solution for this and other adverse canine behaviors is proper training. Your dog needs to understand your commands and willingly comply with them. You can train your Estrela to come to you from a distance and stay by your side no matter what. Then you will be able to leave him in the yard and even walk him off leash without fear of him running off.