Here is the section:
22514. No person shall stop, park, or leave standing any vehicle
within 15 feet of a fire hydrant except as follows:
(a) If the vehicle is attended by a licensed driver who is seated
in the front seat and who can immediately move such vehicle in case
(b) If the local authority adopts an ordinance or resolution
reducing that distance. If the distance is less than 10 feet total
length when measured along the curb or edge of the street, the
distance shall be indicated by signs or markings.
(c) If the vehicle is owned or operated by a fire department and
is clearly marked as a fire department vehicle.
So, if you were within 15' if the hydrant, you are guilty of the offense.
And note the DMV driver's handbook section about "illegal parking":
It's one of the many rules of the road that we are all expected to remember and no signs are required.