If paulie was found shot in his own car with a weaopn that was untraceable, the most the police could determine is that it was a mob hit. They would have no way of knowing which side of the incipient war Paulie was on and they would not know which family had him whacked. Since he was a low level bodyguard for the Corleones, they probably would not have put many resources into that particular murder.

As for the cannoli, Clemenza already had eaten one, and he had his fingerprints on one of the others as well as all over the box.