[Galadriel55]

I still say that it wasn't so much a deception on Saruman's part as simply an everyday - or perhaps slightly more than that - covering of the mines and forges, while not at work. Mines can have a relatively small entrance but span underground for long. Pits and forges can have makeshift roofs, to keep the rain out or just for propriety (though who knows how much of that was going around). And we know from the Ent invasion that there were many shafts that were hidden - probably just by their physical location and nature than by any art of Saruman's. I think Saruman gave his Uruks a state holiday on the day of Gandalf's coming to not make it obvious, but I think Gandalf did not notice too many pits just because those that were on his way were not physically very visible, or only partially so, so you couldn't guess what's really there. Then, safely on top of Orthanc, state holiday was over and Gandalf could see all of it open up for the working day/night. The pits would be more visible from atop, and they'd be open and in use, making their presence more obvious.