The user profile service connection filter excludes any user accounts that don't have a custom 'user ID' attribute populated.
Set-SPProfileServiceApplication $ups -GetNonImportedObjects $true
returns several users without user IDs.
I'm not sure how these users are getting into ups but this means that they can be found in the people search.
I've tried creating a new managed property for the user ID but I don't seem to be able to create a scope rule to require a wildcard ID or exclude blank IDs.
Is there another way to exclude them from search results?