Outdoors, not in a vehicle salutes are then expected and to be returned by the Officer or Warrant Officer that was saluted. LCDR Jerry Maurer - Actually both the Army and Air Force only salute indoors when actually reporting to an Officer, they DO NOT salute for example when passing each other while under a roof inside a building in either service. They also are not supposed to salute the flag unless … Commanders of organizations or detachments that are not a part of a larger formation salute officers of higher grade by bringing the organization or detachment to attention before saluting. Also in the navy because of frequent contact on ship, sailors are …

This means that, like Naval personnel, Marines never salute unless wearing a cover, and thus almost all Marines do not salute when they are indoors.
Marines do not salute indoors when they are uncovered (no hat) But marines under arms and indoors are required to salute.
When in the field under battle or simulated battle conditions, the organization or detachment is not brought to attention.