At the Chicago stop of the TGS signing tour, Brandon Sanderson told fans there was an unnoticed detail, an indication that something big that had started in books 4-6 (he couldn't recall which one exactly) and has continued throughout the series. He goes on to say that he hasn't seen any fan discussion of it yet.

So what is it?

Unfortunately, there seem to be very few clues. In a series as massive and sprawling there are probably hundreds of background details that pass us by as being insignificant at the time, but probably were placed with significant meaning by the authors. Accordingly, there hasn't been much in the way of solid support for any of the possibilities raised.  Some intriguing possibilities include the unseen eyes and related general creepiness of Tel'aran'rhiod, and the current location of the male angreal that Rand lost after Dumai's Wells. But none of these have much evidence behind them. So this one gets a big "We don't know".