Ex-RAF Armourer here, I think it could be one of two things:
Most likely is a Personal Survival Pack (PSP) Automatic Deployment Unit (ADU) Safety Pin. A small Pip Pin with a push button in the head to remove/fit from PSP, with said red warning flag attached by a D Ring. This is used to isolate the ADU when the pack is handled or in transit, but needs to be removed prior to fitting the PSP into the Seat Pan. Purpose of the ADU is to lower the PSP from the ejectee's backside during seat separation. Used on the MB Mk10A (Tornado), 10B (Hawk) and 12 (Harrier GR7/GR9/T10) ejection seats.
I did consider the Emergency Oxygen Bottle Safety Pin, but that is a split pin on a wire with red plastic tag attached- unlikely I think.
I hope this helps!