So there are essentially four RED mandatory holding positions signs
I think you mean potentially mandatory
, right? If ATC didn't explicitly tell you to hold short of anything other than the runway, then it shouldn't be mandatory.
Reference AIM (4-3-18(8)):
8. If a pilot is expected to hold short of a runway approach (“APPCH”) area or ILS holding position (see FIG 2−3−15, Taxiways Located in Runway Approach Area), ATC will issue instructions.
Reference 7110.65W (3-7-5(a)):
3−7−5. PRECISION APPROACH CRITICAL
a. ILS critical area dimensions are described in FAA Order 6750.16, Siting Criteria for Instrument Landing Systems. Aircraft and vehicle access to the ILS critical area must be controlled to ensure the integrity of ILS course signals whenever conditions are less than reported ceiling 800 feet or visibility less than 2 miles. Do not authorize vehicles/aircraft to operate in or over the critical area, except as specified in subparagraph a1, whenever an arriving aircraft is inside the ILS outer marker (OM) or the fix used in lieu of the OM unless the arriving aircraft has
reported the runway in sight or is circling to land on another runway.
HOLD SHORT OF (runway) ILS CRITICAL AREA.