Most honest people don't take kindly to being called a thief. If your shop is honest you will not win any points by accusing them of intentionally stealing your tire.

Instead do yourself a favor and consider the following scenario - The tire changer is likely the most inexperienced person in the shop. He can make a mistake at any time and during the confusion of a power failure it is all the more likely. The person simply forgot to return the spare. As to why he didn't fess up after the fact I don't know. Maybe the tire got rolled over to the repair area and is still sitting there. Maybe they don't remember your car/rim combination well enough to call you up and say they have your spare. I mean if they up sold you a tire rotation they probably try to sell most everyone.

Sure they could have stole your tire on purpose. However if they did they certainly won't admit it weeks later.

Point being, I suggest you start your conversation with the shop by giving them a chance to correct an honest mistake. Whereas if you go the "Stealer-ship" route it will only piss of an honest shop and fall on the deaf ears of a clip joint.

Good luck.