The [Edit In] command for External Editors first creates a file (type determined in preferences) in the Catalog (you may not see it before the editor 'opens'), then sends it to the other editor for you to edit and [Save]. Do not use [Save As].
Not being visible in the Library could be-
-A Filter that is not showing the new file.
-A Smart Collection that does not show the new file.
-The Sort Order places the new file in a different location to its 'original'.
-Stacking of the Original and Derivatives.
Have you searched the Library for the Filename-edit?