Its in the Group Orders. I'm not sure which one (since I have no access to them from home) but look around the 200 series dealing with duties. The problems with MPs pulling base duties is that by virtue of 156 NDA we are always "on duty" in that it outlines our authorities but it does not specify "while on duty" or any such limitation. Add to that 2(g) CCC which states that we are peace officers while appointed under NDA 156 and you can see where its not as though we can turn our status on and off. Should an MP be required to act as Base Duty Cpl and that MP gets sent to the Mess to deal with an unruly patron....are they there are Base Duty Cpl to perform those duties or are they there for law enforcement duties? The person who allows you to be put on Base Duty may say you are not acting as an MP during that time but the NDA states otherwise.
I remember back when I did my JLC I was told by the staff that I was not there as an MP and therefore I was not in a position to comment on actions of some of the DS. They had also told the medics on the course that they were not medics while there, they were students, and the DS would look after any frostbite issues or minor injuries on the course over the winter. Well, I think by the time the Court Martial was over because two students had lost toes due to frostbite they had a different view of when an MP has the authority to get involved.
In any event, start with the low 200 series of group orders.
