They don't identify themselves by signature, it's voluntary that they even identify themselves actually. Your comment helped me correct a slight inaccuracy, thank you!
In the "For the technical reader" section on Mining distribution I fixed the paragraph that mentioned "signature". It now reads: "Miners are identified by their coinbase tag, a free-text field the block's miner writes into its own coinbase transaction. That miner may be a pool or someone mining solo."
As for when they get tagged in my report rather than show up unidentified, it's a bit random. I monitor that field manually. I read the report every morning and notice the unidentified and then add it to the query when I get around to it (if its identifiable, meaning: have they put their name in the free text field of the coinbase transaction?
I like doing that part manually for now because it keeps my eye on the mining sphere.