If Michael had suspected Rocco of complicity in the Tahoe attack, he would have had him killed ASAP. Unlike Fredo, Rocco would have been too dangerous to let live. Rocco had simply outlived his usefulness to Michael, and he was expendable.
Johnny Ola and his men arrived at the compound conspicuously, and no doubt left conspicuously. Ola would have made sure Michael saw his men and him leave, in order to divert suspicion to Frankie.