Final answer:
The Sentry caught Antigone red-handed as she was performing the burial rites for her brother Polyneices, directly violating Creon's decree that Polyneices should not be buried.
Step-by-step explanation:
The Sentry discovered Antigone was the one who buried her brother Polyneices by directly catching her in the act. In Sophocles' play Antigone, Creon, the king of Thebes, has decreed that Polyneices, who died leading an attack against the city, shall not be granted a proper burial. Defying this edict, Antigone performs the burial rituals for her brother. However, the Sentry, who was set to watch over the body of Polyneices to prevent anyone from burying him, spots Antigone as she covers Polyneices' body with earth and performs the burial rites.
The encounter between the Sentry and Antigone is a critical moment in the play. It demonstrates Antigone's unwavering commitment to familial duty and the divine laws over the edicts of man, even under the threat of death. Her actions enrage Creon, which sets a tragic chain of events in motion.