Ghost ability makes the unit 100% invisible, regardless of if it attacks, casts an ability, etc, it remains invisible/untargetable unless the ability is removed. (Or something detects them, of course)
And I think Ghost (Visible) makes the unit untargetable but seeable. So that could be an alternative to Locust if you want it to be unselectable but still have collision and be a unit that can take orders.