Dixie,
If you want an answer to where the DoD police have jurisdiction, just ask a St. Marys Sheriff. They will now exactly what their jurisdiction is and any areas under an MOU agreement.
I based my answer to you before, that the DoD officer was not within his jurisdiction because of the following:
1) a Post Office, while still a piece of Federal Property, does not fall under
the Department of Defense.
2) In Charles County where I work, we have a U.S. Post Office just 75 yards
outside of the gate at Indian Head NOS and DoD has no authority on
that property. CCSO handles all incidents at that location, unless the
U.S. Postal Inspectors choose to get involved.
Also, you refer to the officer "running" you. Thats is nothing more than a warrant check through Law Enforcement data bases. Thats not a detention or an indication you have done anything wrong.
Now for the other side of the coin.
If the DoD officer was traveling (I assume from your description that he was in some kind of police car) from different points that his job requires, and came past the post office, and observed you, and anybody you were with, at a late hour. It is certainly possible that your actions and presence at the post officer were suspicious. It would not be out of line for him to stop and, determine if some type of criminal activity was occuring. If he did determined criminal activity was in fact occuring, he could detain the offender, until the St. Marys County Sheriff arrived, who would have handled the incident. The type of criminal activity I am refering to here would be someone breaking into the building or causing a destruction of property.
You need to remember that you know you were not doing anything wrong, but a Policeman driving by a government building, late at night, long past closing time, may perceive just your presence as suspicious and out of place.
In this case, he stopped and asked some basic questions, determined you were not up to any criminal activity based on those questions, and went on his way. This may be what happened in your case.
Contrary to some folks ideas, the police are allowed to talk to citizens and ask questions. Their is no law that exists that prohibits the police from approaching you and asking you who you are and what you are doing, when you are in public. If you choose the option not to answer, then things may go several different directions and it is possible the officer(s) may engage further contact. Shining a flashlight on someone by the police at night during this type of contact does not violate any civil or constitutional rights. *I noted you were particularly offended by that.
Dixie, the bottom line here is if you feel the officer acted improperly, DoD does have a complaint process just like other police agencies. I would encourage you to use it, if you feel something was unappropriate. If you are just curious about the juristiction issue, to get a accurate answer, ask (or call) any St. Marys County Officer. They can tell you, because they won't handle incidents on property if it belongs to DoD unless they have an MOU in place.
Again, I hope I have shed some light on your question and maybe helped you understand what may have happened.