Who places the daggers with the murdered servants in Macbeth?

Macduff greets Macbeth's announcement that he has killed the grooms with the line "Wherefore did you so?" Macduff seems to think that was a peculiar thing to do, since he asks why Macbeth did it. Macbeth has a ready answer, but then in the next scene Macduff chooses not to attend Macbeth's coronation, so maybe he was not convinced by the answer.