Summary
When accessing non-existent pages of the web site the user is referred to /system/404 or /system/emptysearch, according to the context. But these pages don't respect the HTTP specification, returning status "200 OK" instead of "404 Not found", specifying that the desired content could not be found.
One of the relevant cases is the correct interpretation of Google when indexing the page, so pages with content not found should not be indexed.
Simulation
Access any non-existent page from your store, or directly enter /system/404 or /sistema/emptysearch, and then check the HTTP response status of the request, which will be "200 OK".
Workaround
In case of pages being recurrently indexed by Google, the best option is to create a URL mapping, directing the content of the non-existent page to a relevant one.