You have a point there, but I think Michaels suspicion arose when the shooting took place in the middle of his own compound right in his bedroom. The enemy had to be an insider to easely squeeze through those gates and make his way straight to his home. Its a common lesson taught to us in the Godfather series that your worst enemy is always close to you, he's an old friend or a family member, even Michael knew so since he warned Vincent in GF3 about that. (How he had to chose between the life or Mary, because when his enemies would come they'd come after her or everything close to him, and they'd be close to him too)

Any thoughts about this?