Nope. Some buy into the philosophy that if he engaged in any casual conversation, go ahead and inform for your own benefit (ie just in case) but you certainly don't have to. Now, if that conversations turns to a legit law-enforcement purpose, you would be required to inform.
Interesting fodder for a case study: What if you are casually engaged in conversation with LE, suddenly the cop thinks you resemble a suspect discussed in a morning brief, they begin asking you questions to that end, you inform and the cops now says you didn't promptly inform despite the previous conversational composition... Wonder if it's ever happened.
NRA Life Member
"Who are the militia? Are they not ourselves? Is it feared, then, that we shall turn our arms each man against his own bosom? Congress shall have no power to disarm the militia. Their swords, and every other terrible implement of the soldier, are the birth-right of an American ... The unlimited power of the sword is not in the hands of either the federal or state governments, but where I trust in God it will ever remain, in the hands of the People."