When you create a new module or topic in Content, it is by default visible to students. You can then choose to hide it. But if you do not want your module or topic to be visible to students for even a moment, or for students to receive a notification that something new has been created, set up a module for yourself (call it something like Instructor resources) and make it hidden to use as a place to create material that needs to be automatically hidden from students.
In that hidden module, create a submodule. Hide it (select the chevron beside the name and select Hide from users) then in the Table of Contents, drag and drop it to where you would like it to go at the module level.
You can do the same for a topic that you would like to be hidden in a module that is already visible. Create the topic in your hidden module; hide the topic (select the chevron beside the name and select Hide from users); then select the chevron beside the name of the topic; select Move to; select the module you would like it to be in; and select Move.