How is the murder of King Duncan discovered? Who discovers it?
In Act III, scene III of "Macbeth," Macduff and Lennox arrive at the castle in Inverness. With portentous irony the porter who comes to answer the door speaks of "equivocators" and speaks of the castle as being "too cold for hell." When Macduff and Lennox enter, the porter jokes with Macduff about his [the porter's]drinking and carousing the night before. With unconscious pun, Macduff replies, "I believe drink gave thee the lie last night."
Then, Macbeth enters and discusses the violent storm the previous night while Macduff leaves only to discover the body of King Duncan. He rushes back
O horror, horror, horror! Tongue nor heart/Cannot conceive nor name thee....Confusion now hath made his materpiece./Most sacrilegious murder hath broke ope/The Lord's anointed temple, and stole thence/The life o'th'building..../Ring the alarum bell. Murder and Treason!(65-75)
At this point Lady Macbeth enters, acting shocked: "What, in our house?" (88), and faints. Macbeth, too, feigns shock and grief:
Had I but died an hour before this chance,/I had lived a blessed time; for from this instant/There's nothing serious in mortality;/All is but toys. (91-95)
However, in spite of the supposed shock of the Lord and Lady Macbeth, Donalbain and Malcolm fear foul play has been committed by someone close to them:
Where we are/there's daggers in men's smiles; the nea'er in blood/ The nearer bloody (127-129)
Of course, the dramatic irony of this remark is not missed by Shakespeare's audience.