The only official confirmation of this launch came from the report given by the commander of the Rocket Forces - Sergey Karakayev said that there were eight ICBM launches in 2013. The open record, however, showed only seven, so one launch was missing. As it turned out, there was a reason it was unreported. The launch, conducted around 26 September 2013 from the Baykonur test site involved an UR-100NUTTH/SS-19 missile carrying the infamous "hypersonic" warhead. As I understand, the missile worked well, but the payload failed, so no announcement was made.
It's interesting that it's soon ten years since the "hypersonic warhead" first made an appearance (that was in February 2004, also in Baykonur and on UR-100NUTTH). That time it wasn't quite successful as well.
It's not quite fair to say that the test went completely unreported - there was a very brief word about a failure at some point before September 30, but no details were given. Now we know that a UR-100NUTTH missile was involved and have a slightly better date (it could still be a day or so off, though).
UPDATE: Not so fast, my readers tell me - the "hypersonic test" was a launch of a test vehicle from a Tu-22M3 aircraft. Well, it's certainly possible. I hope more information will come out eventually.
UPDATE 12/17/2013: It appears that an ICBM test did take place - it was probably the third "development" launch in 2013 reported by Sergei Karakayev. An intriguing possibility was mentioned in one of the NK discussions - the missile could have been launched from Dombarovskiy. We know that some construction (and "hypersonic"-related one at that) there has been underway for some time, so it is not entirely impossible.