Yup - that's a pile of nice, brand-spankin'-new 1x3s. They were hidden behind the plywood & cardboard under the Old Saybrook section of the layout.
And I had literally just cut open the (now-extra) bundle I bought just about 30 minutes before discovering this pile.
So I guess the take-away is to try and keep better track of my materials. Not hiding them behind other things would be a great start.
At least now I don't have to worry about running out of 1x3s. Heck, I might even have enough to start another expansion...