There was once "core unlocking" on AMD Phenom, I think. Intel has never had that.
An Intel unlocked CPU is marked with a "k" at the end - 4790K. The only unlocked piece in these CPU's is the multiplier.
What you were saying about the hidden cores, that's called "Hyperthreading", the CPU's core is split in two, so it can do more tasks at the same time. AMD's ZEN will also have this ability on some of their CPU's, I think it's called "Simultaneous Multithreading".