English: “If you hear” — and not that you must go seeking out the matter; one might have thought it may be neglected, therefore the verse states “and you shall inquire and investigate and ask thoroughly.”
On the daf / in context: The language “if you hear” indicates that one is not obligated to search actively for an ir ha-nidachas, yet once rumor reaches him he must investigate fully.
Type of Learning
Derash on the verse (Sifrei) — the duty of investigation arises only upon hearing, not by active search.
What Is Learned
One is not required to hunt for an ir ha-nidachas, but once one hears of it one must inquire thoroughly.