Hi Francisco, it's highly likely that someone simply shared their project with you. For example, if someone makes you an Editor on project "ABC", then project "ABC" will show up in your projects list and you will be able to access project "ABC". This doesn't change anything about the access level anyone else has to your projects.
You can check who has access to projects that you're an Owner of by going to the "IAM and Admin" page for a particular project.