I've recently upgraded my iMac to OSX Mavericks. I'm finding that when I press Control-Shift-N something is immediately locking my screen and making it dark. I can still hear my streaming audio playing, so I guess it's not logging me out. I've looked carefully through the list of keyboard shortcuts in System Preferences, and that key combo isn't mentioned anywhere.

Is there any place I can get a list of all active keyboard shortcuts? Is there any terminal command I can run to see which application has taken over this keypress combination? It's really annoying because I habitually use that combo in PHPStorm! (And yes, I have checked the keymaps there too.)