It's a translucent black plastic, like the kind they screen print the gauge warnings on and back-light. Not sure where you would get a replacement piece from.... I'm wondering if they happen to be cut-outs from the dash gauge assembly myself (hey - If ford used to re-use the boxes the auto parts shipped in for Model T floorboards... why should Nissan waste those things?)

Just curious... have you pulled up the seats and checked to make sure it isn't lying there? Or maybe it slipped into the seat crevices...

You can get black abs plastic and cut it to fit, but I would prefer the factory look myself.
-=RB