This idiom emphasizes that something was very close physically but missed being noticed.

Physical Closeness

It implies that someone failed to see the obvious, as if they were blind to it.

Metaphorical Blindness

Usually refers to physical items, not abstract concepts or emotions.

Implies that something was done secretly in someone's presence without them realizing.

The idiom can indicate a form of deception which is carried out openly yet unnoticed.

The person is in the vicinity and could have seen the action but remained oblivious.