Pulling a fire alarm will not activate an automatic sprinkler system. Sprinklers will only activate when each sprinkler head senses fire in its vicinity. This applies the water only to the fire and doesn't waste it everywhere else.
In the beginning when Jordan Hester's social security number shows up on the computer, Mr Finch says that this person has no drivers license, no ID and no photos on record as if the person doesn't exist or lives completely off grid. Later on it is revealed that there is a case of identity theft. In that case, even though the fake one might not, the original Jordan Hester whose identity got stolen would have some kind of ID and photos.