hear the grass grow

English

hear the grass grow (third-person singular simple present hears the grass grow, present participle hearing the grass grow, simple past and past participle heard the grass grow)

(idiomatic) To have an extremely sensitive sense of hearing.
- 1855, Thomas Bulfinch, Bulfinch's Mythology: The Age of Fable, ch. 31 "Northern Mythology",
  Heimdall is the watchman of the gods. . . . So acute is his ear that no sound escapes him, for he can even hear the grass grow and the wool on a sheep's back.
(idiomatic) To hear very well due to being in an exceptionally quiet environment.
- 2005 August 28, Tony Francis, “St Neots dreams dashed along the road to Cup final”, in telegraph.co.uk, retrieved 1 Aug. 2008:
  A suburban arena that was so sepulchral you could hear the grass grow.
(idiomatic) To be very aware or discerning; to pay attention to every small detail.
- 1995, Hans-Peter Schwarz, Konrad Adenauer: a German politician and statesman, Berghahn Books, →ISBN, page 397:
  He was quick to appreciate Blankenhorn's engaging personality and intelligent use of his skills. Chief among these was a remarkable ability to keep track of developments—to ‘hear the grass grow’.

which sense?