The Eleventh Hour: Prisoner Zero exists just out of the corner of one's eye; the only safety is in not seeing it.
The Beast Below: Willful ignorance of the existence of the starwhale and what the people of Starship UK have been doing to it.
Victory of the Daleks: Amy cannot remember the Daleks, when it's unthinkable that she wouldn't know about them.
Time of the Angels (so far, halfway in): An entire lifeform that exists only when you're not looking at it.
And no one sees the omnipresent cracks.
This season seems to be very much about things unseen.