When I was growing up in Queens, one year a single daffodil appeared under our forsythia bush. We had planted no daffodils, anywhere in the yard. Our best guess was that an enterprising squirrel had found the bulb, dug it up, and reburied it. (I read somewhere that squirrels won't eat daffodils, but maybe that's why this one was left alone.) Or maybe it, and your crocus, actually grew from seed.

My partner cattitude feels strongly that it's not spring until he has seen a crocus and a robin, but snowdrops often appear before crocuses.