Similar story; One of my good friends, collected some specimens in North Dakota, brought them home, put them in a corner in the in the garage and forgot about them all winter. In the spring he rediscovered them, they looked like they were all dead but he planted them anyway, they all grew. That's where I got my pink flowering ones from.Someone may have tossed it there or it came in with other stuff at some point. They are super tough plants. When I was in high school, a co-worker who had pretty much about a 75% cacti covering on his property gave me a bunch of cuttings I put in a 5 gallon bucket. I stuck the bucket under the back porch and completely forgot about them. About 4 or 5 years later I stumbled upon them and most were still alive. They were shaded and without water all that time. I was amazed.