Fire sprinklers have two requirements: to be able to turn on immediately if they're ever needed, and to dispense something capable of extinguishing a fire. In order to accomplish this, the pipes that feed them are constantly, 24/7, full of water, providing constant pressure on the sprinkler head to be ready to feed it with water in case it ever needs to go off. These water pipes are generally not used for anything else, so the water does not tend to circulate. In fact, there's usually a sensor in them that detects if the water is flowing (and thus if any sprinklers have been triggered, providing somewhere for it to go) and activates the building's fire alarm. When a fire sprinkler goes off, the water that comes out has been sitting in that pipe (an iron pipe if you're lucky, a lead pipe if you're not) basically since the building was built.

That stuff is NAS-T.