Did it come with the car originally or did the original owners buy it?
Given that there's a wheel well to accommodate it under the boot floor I'd say it came with the car. Looking at skoda.ie a full size steel wheel is listed as standard equipment for the basic model of the current range of Octavias. I'm not sure that not being listed under other variants necessarily means it doesn't come with those. Mine differs from the current basic model insofar as it has alloys, a center armrest with storage box, various buttons on the steering wheel, automatic transmission, cruise control and such like yet it was clearly manufactured to accommodate a full size (steel) spare wheel.