Marines don't have to be in uniform to be recognized as Marines. These are the distinguishing characteristics: very short haircuts, relative youth, obvious good physical condition, military bearing, always at least two together, and, almost always, politeness. There are Marines in the Hawk & Dove every night. It is a place where "my little pony" Capitol Hill staffers go to meet real men.
Perhaps you are correct. I still suspect they were in uniform.