Random Scenario.....?

Depends. Would he be the only survivor? If not, then the police would definitely find out. It's pretty hard not to recognize a grenade after it's started falling from the third floor. The confetti would fall off or separate so that it could be seen. If he were the only survivor, it depends even more. How good an actor is he? If he were really good, the police might not find out. Did the police make him take a lie detector test? Yes, he could be found out. How was is hand positioned after he got blown up? Did he keep the ring in his hand? Was there security footage? It all depends on how well he is at hiding it.