On Campbell St., west of Lansdale Ave., find stairs leading down to the Hillside Underground Entrance. There is a painting here "Sunrise", needed for a side quest that you will get later, "The Art Collector". There are also some supplies and weapons along the tunnels. The underground tunnels also allow you travel to other neighborhoods more quickly, but those exits will most likely be locked for you at this stage of the game, and will unlock later after you complete a certain quest. It is easy to get lost in this labyrinth of underground tunnels, so try and remember which way you came from.

After exploring the underground entrance, go outside and wander a bit down the streets. Look for police patrol cars, and approach one of them as it comes by. To your surprise, some Screamers will come out of it. Either attack them or run. Either way, you have unlocked a side quest, "All Points Bulletin", and a couple of new tasks:

Stop the Patrol Cars
Police cars everywhere. They don't feel like normal cops. Something's very wrong here. What kind of town is this?

and also:

Find the police patrol dispatcher
If the town is deserted, who's dispatching the patrol cars?

Go to the Hillside Police Station on Campbell St between Mc Cammon St and Straub St. The main entrance to the police station is boarded up, but go around and find a gap in the wall. To the left, on the floor, pick up a police department vehicle theft report, which will be added to your journal under the "Mysteries" section.

Cross the narrow beam and reach the dispatch room. There is a flashlight here if you need one, as well as a first aid kit. Go to the main desk and use the radio dispatcher (Note, if you have not met a police patrol outside and unlocked the "stop patrol cars" and "find police dispatcher" objectives, you will not be given the option to use this radio)

This is a PUZZLE. You have to recall all four patrol cars. On the radio, there are 4 letters (A,B,C,D) representing the four police patrol cars, as well as 9 digits, representing a police car's callsign.

Downpour Police Dispatcher for All Points Bulletin Sidequest

Police Patrol Puzzle Solution
Notice the blackboard on the wall. It will have the call signs for all 4 police units. But a part of each callsign has been erased. You can either guess the missing numbers using trial and error, or notice that the numbers are in a sequence (From D to A, then from 3 to 6, then from 7 to 4,  then from 5 to 8) or you can go look for the patrol cars and read the call sign written on each patrol car itself, although that is the most dangerous way to solve this puzzle since the police cars are full of Screamers.

Note: The harder the puzzle difficulty is set to, more parts of the callsigns on the blackboard will have been erased.

The police dispatch codes are:

Enter each code into the dispatcher device. This will complete the All Points Bulletin Side Quest and will unlock an achievement/trophy, "Calling All Cars".

Before leaving the police station, break the barricaded door near the police dispatcher, and enter the hallway to find another mystery object. There is also a radio there. Turn it on. You hear a voice calling for help. She says he is at the station in the old clock tower. There is also another painting in this area, "Despair", needed for the Art Collector quest.