> How long was it visible for? And no, "briefly" doesn't cut it.

I don't know how long exactly, but in theory you can subscribe to a stream of ALL events happening at GitHub by fetching from this endpoint: https://api.github.com/events

With these events you know what new repositories are created and what changes are pushed, so you can fetch every new change. Once something gets pushed to a public repository, it's very likely that some spider will have fetched it within a few minutes.