1. Go to Notes and create new note without а date.
2. Choose 'Selection' in Details Pane and select created note.
3. You won't see the note in Details Pane (instead of note you will see Today summary). Everything is OK if note is pinned to date.
To see the note without a date in Details Pane you should select the note, switch to another tab (Tasks or Events) and switch back to Notes.