The repository itself has a default volume.  Anything not specified to go to a different volume, will go to that default volume.

Migrating documents does move them from one volume to another, it does not copy.

Changing the default volume for a folder only impacts new documents created in that folder going forward, it doesn't impact items already in the folder, and it doesn't prevent users from selecting a different volume manually.

You can see how many items are in a volume from the Volumes page of the Administration Console.  By selecting a volume, opening Properties, and browsing to the Statistics tab.  This can take several minutes depending on how large your repository is.

You can also search for contents of a volume within the Client or within Workflow.  From the search screen on the Client, you can select "Within Volume" to search within a volume.  If you are doing search syntax, the syntax is like this: 

({LF:VolId="1"})

If you want to use Search Syntax and don't know the volume ID, you can get it by searching first using the "Within Volume" option while "Search Syntax" is enabled (but not marked as included), or you can find the ID on the Volumes page of the Administration Console.

Side note - after a very large volume clean-up project (we had millions of documents, just hanging out in the DEFAULT volume) a couple years ago, I set-up multiple workflows that run nightly to identify items that are not in their expected volume, and migrate them to where they are supposed to be.  That way, things are always consistently in the volume they should be, regardless of whether or not staff are following the guidelines regarding volumes.  I didn't try to get rid of the DEFAULT volume, I just worked to ensure all my Workflows migrated documents into their correct volume, that all folders had a default volume selected, and all Forms had a volume selected - along with the workflows to migrate items.  Thus nothing stays in the DEFAULT volume for long.