Sometimes I experience that closing a document leaves the empty "window" open.
I cannot recreate this now, but it happened in the past: for example when I choose "edit in designer..." in the retail version.
Now it happened in the 523 beta with a Document I had open for maybe 3 days and then saved and pressed Cmd-W to close it.
(Mac Mojave 10.14.6, Dual Monitors Separated Mode)