Depending on an Environments setting it is possible to see Hub items across your different Collaborative Spaces. For example a user might be able to see all of their tasks that belong to them regardless of the Space they are currently logged into.
Here are a few examples of when a user would see a Task that exists outside of the Space they are currently logged into.
The first example is when a task is assigned to a user and a collaborative space is public. When a Space is public it makes everything assigned to that user visible. To avoid this make sure a Space is made private or protected. It is not possible to transition a Space from Public to Private.
There is a new Hub setting that restricts the visibility across Spaces. more information on that can be found here.
Another example is if a user has been added as a Service account. This user can see everything across all Spaces regardless of its Privacy. If the user is able to see items that are part of other Spaces, check to see if they have a service account. Best practice is not add users to the Service Account Groups in Hub.
It is possible to see items across Spaces. These two examples are important to keep in mind when this occurs. Make sure to check the settings to ensure the correct visibility of these items in your environment.