If you haven’t docked the Open menu in Notes 8 on the left you want to be able to access it using the keyboard. As always the shortcut list is to the rescue. Ctrl-Shift-L shows a list of all shortcuts and in that list you find “Open list” which is what you want. The shortcut for “Open list” is Alt-B so put away that mouse and use the keyboard.
Update: Just relealized that it works just as well with the Open list docked.