Theres no such thing as an anonymous nuclear explosion. Each plant that manufactures nuclear material leaves behind, in that material, a unique mix of chemical and radioactive substances. Sort of a fingerprint. Over the decades, the major intelligence agencies (especially those in the U.S. and Russia) have compiled lists of these fingerprints. If a bomb goes off using unrecognized material, you know you are dealing with one someone who has been trying to keep their nuclear weapons work secret. Thats a short list of suspects, and its unlikely that the culprit will stay unidentified for long.