Key "cut" variations are not infinite, so they can be repeated at some point in a vehicle's history. Old Honda motorcycles are notorious for this; they recycled the key codes constantly over the millions of bikes made, so it is fairly easy to find a used replacement from a box of old keys. What's amazing is the old spare you had actually matches your current ignition lock. And actually the key might not necessarily be an exact match. As the key and the lock cylinder wears, the profiles of either/both become less defined. This makes them less "unique", which is the principle of a car thief's "bump key". But I would not worry about it being more of a theft risk, if a thief wants to steal you car he will regardless of the condition of the lock. I see no reason not to just use that key "as is".
Oddly I had a similar experience. I have a motor-home that I use to haul race vehicles to tracks for weekend events (and motor-homes have lots of locks on them). On one such trip I drove for about 4 or 5 hours, getting roughly half way there, when I stopped for a gas break. I was checking everything over when I discovered I had left one of the needed keys (to gas up) at home. I was too low on gas to get far so needed to find a way to open the lock. About to call a lock-smith when a guy parked next to me started a conversation about the race car. I mentioned my predicament and he said "will it help to try some of my keys?", as he pulled out a key ring with about a dozen various keys on it. Although my instinct said don't waste my time, I went ahead and started trying his keys. And yes, one of them worked. Same reaction you had.
