1,A chicken I bought yesterday is missing.
2,Her father was shot by a man trying to rob his retail store.
Theoretically, a phrase that refers to something identified uniquely should take a definite article.
Like scenario 1, "A chicken." indicates I bought more than one chicken yesterday. If I only bought one chicken, this sentence should be rephrased as "The " chicken I bought yesterday is missing."
But Scenario 2, many people feel that "a" doesn't exclude the possibility that there was only one man robbing. How is this happening?