I would suggest using the following native
GetClosestObjectOfType
This native does all the work for you and returns the closest object that you want to find.
So in your case, the object you want to be searching for is prop_fire_hydrant_2. I believe the same fire hydrant is used across the entire map, if not, you can simply search for multiple objects instead of just the model mentioned above.
For a full list of objects (and their corresponding hashes): click here
For more information about this native: click here