How was Pippin able to talk about his experience with the fellowship to Denethor for an hour without giving any hint about the ring?

Before they enter the great hall at Minas Tirith, Gandalf tells Pippin to 'leave quiet the matter of Frodo's errand', for obvious reasons. But Denethor has Pippin, in Gandalf's presence, give an hour-long account of his experience with the fellowship. How can Pippin have possibly avoided giving any hint of the ring - without which there would be no explanation for the journey of the fellowship?