While there isn't a NW central lost & found number, there is a central baggage service number I was given for lost/forgotten item claims, but I found it to be totally worthless.

Next time try the WorldClubs desk (assuming you have access and there is one) at whatever airport you may be at. I also left an item on the plane a while back, and after having, like you, no luck trying various avenues, dejectedly made my way to the WC where I mentioned my forgotten item to the agents at the WorldClubs counter. The ladies there made a few phone calls to the right people who were able to locate my lost item and had it held for me for my return through the same airport. ^