Are you sure it was a shortcut key combo and that it created a new layered document. I know of shortcuts that will dupe a layer and create new layers and stamp all visible layers into a layer however I do not know of a standard shortcut that creates new layer documents. Shortcuts can be added and assigned to commands and actions etc. These can dupe the document actions and scripts can do many thing. Are you sure what you remember was a standard Photoshop feature.
The only default shortcut i can think of is Shift+Ctrl+Alt+E (Shift+Cmd+Option+E) for stamp visible to a new layer.
Before cs2 one had to make the blank layer before using the shortcut , so that would have been Ctrl+Shift+N and then Shift+Ctrl+Alt+E.
Or maybe Shift+Ctrl+J for new layer via cut
After reading your post again and assuming your image is in grayscale mode this might be what your thinking of:
Ctrl+Alt+1 selects all the white areas (Ctrl+Alt+3 on photoshop cs4-cs6)
Ctrl+Shift+I inverts the selection so the black lines are selected
Ctrl+J Layer via copy sends the black lines to a separate layer
What version of photoshop and operating system are you using now?