In ‘The Adventure of Silver Blaze’, Sherlock Holmes mentions ‘the curious incident of the dog in the night-time’. ‘But the dog did nothing in the night-time,’ argues Inspector Gregory. ‘That was the curious incident,’ replies Holmes.

You never hear anyone say: ‘We finally stumbled across a charming little petrol station nestling among the trees’

Along with Donald Rumsfeld’s ‘Unknown unknowns’, this is perhaps the most famous example of what you might call ‘perceptual asymmetry’.