macduff's subtle suspicion of macbeth

In Act II, Scene Four, Ross and an old man discuss the unnatural events that have taken place recently: days are as dark as nights, owls hunt falcons, and Duncan's horses have gone mad and eaten each other. When Macduff enters, Ross asks whether the culprit has been discovered. Macduff tells him that the bodyguards killed the king. The hasty flight on the part of Malcolm and Donalbain, however, has also cast suspicion on the two sons as well. Ross comments that Macbeth will surely be named the next king, to which Macduff responds that he has already been named and has gone to Scone to be crowned.