Unexpected visitor

The other night, I'd just got home from work, and was sitting down to dinner when I noticed something flutter by my feet. And then Rocky pounced.

At first I thought it was a little house moth (we see a few now and again), but as Rocky moved his paws, I noticed it was a fair bit bigger than our usual moths. I bent down to rescue it from Rocky and...

... discovered it was one of our favourite little White Cabbage Butterflies. I have no idea where he came from. It's entirely possible he flew in while I was bringing in the groceries...

...but we usually don't see them past October. We had a really mild November, but we have had frost, which I think would have killed them all.

The only thing I can think is that maybe there as a cocoon hiding on my one of house plants. I had no choice but to put him outside. I doubt I could have kept him alive throughout the winter...especially with the mighty butterfly hunter in the house...