Only guessing, but...
Funny thing is (especially considering my background), I don't recall if the building is sprinklered or not. Too busy looking at the computer screen (courtesy of the Gates Foundation) to look up; I guess I'v finally gotten out of the habit.
Coulda been a sprinkler branch; coulda been a branch to an exterior hose bibb. Probably wasn't the domestic water main - I know where the toilet rooms are, of course, and assume the water service enters the building right nearby, not at the other end of the same wall (no closet in that back corner to hide the meter in).
Always wanted to do forensic engineering...