Well, Google finally confirmed this morning that there was a core ranking change but unrelated to Penguin - as we expected.
It broke this morning with Zineb from Google posting in French:
That means there was no name [for the update]. It is simply one of frequent quality updates.
Gary Illyes confirmed the translation and what happened:
And John Mueller did also:
@dawnieando It's not penguin; not "confirmed-ish" just "confirmed" :-).— John Mueller (@JohnMu) January 12, 2016
We had several core updates that Google has confirmed in the past, so this is one of the rare ones that Google has indeed confirmed.