I was about to give the following advice:
Pick the entire carpet up, put it in the bathtub, douse with gasoline and burn that shit. Then turn on the water and wait for the ash to rise to the water surface. All that's left is to scour the bathtub bottom for the screw!
But I see you already found it. I once found an entire chocolate bar, still in wrap, inside one of the couch folds. It's been sitting there for about 10 years and when I opened it had morphed into a perfect turd, with the most even peanuts distribution you can imagine.