This is a list of every random event on the map. Each event occurs during the intermissions of their mentioned waves. For example, if an event mentions it is only possible on Wave 3, it is only possible in the intermission between Wave 3 and Wave 4.
Stop reading this if you don't want to be spoiled!

If you want to force-trigger any of these, type "ent_fire (relay name) trigger" in the developer console with sv_cheats set to 1. Relay names are provided.


Helicopter Pass
random_skybox_pass_relay

A HL2 hunter-chopper flies across the skybox and over the main map. It is estimated to spawn and despawn over the course of about 2 minutes, lasting throughout the entire intermission. Not a NPC.

Possible on: Wave 1, Wave 4
Repeatable


Radio Chatter
random_radio1_relay

prison_soldier_bunker2.wav plays in the Factory 45 "control room".

Possible on: Wave 1, Wave 2
If triggered a second time, random_gunfire1_relay will play instead, representing the Hunter 6 squad mentioned in the transmission.


Garage Door Malfunction
random_gardoor_shut_relay

A garage door in Factory 45 suddenly closes and the button becomes inoperable, leaving the back door the only(?) way in for humans.

Possible on: Wave 2


Random Explosions 1
random_explosions1_relay

Random explosions are heard in the direction of the city.


Possible on: Wave 2


Random Gunfire
random_gunfire1_relay

A shootout takes place northwest of the map. Are they shooting at zombies, or each other? If you're close enough, you can hear them die about 30 seconds in.
These can easily be mistaken for NPCs, but they're really just a few env_gunfire, logic_case, and ambient_generic entities. (which means they're harmless and not NPCs)

Possible on: Wave 2


Sonic Boom
random_sonicboom_relay

My poor excuse for the sound of a sonic boom (portal_beam_shoot5.wav) plays and every window in the map is destroyed.

Possible on: Wave 2, Wave 4
If triggered a second time, random_water_relay will play instead to help undermine the OP spots.


The Great Door Uplift
random_uplift_relay

Every garage door in Factory 17 suddenly opens, forcing any humans in the main room to fall back to their barricade, if they have one.

Possible on: Wave 3, Wave 4, Wave 5
If triggered any more times, random_canister1_relay will play instead to help undermine the OP spots.


Random Explosions 2
random_explosions2_relay

More random explosions, this time slightly closer to the main map.

Possible on: Wave 3


Airdrop 1
random_airdrop1_relay

Three flares fall from the sky, followed by a crate containing some goodies.

Possible on: Wave 3


Open Factory 52
random_openfactory6_relay

Opens a small reception room with nothing in it.

Possible on: Wave 3


Random Canisters 2
random_canister2_relay

Three empty headcrab canisters hit some unreachable areas.

Possible on: Wave 2, Wave 4
If triggered a second time, nothing will happen at all.


Random Canisters 3
random_canister3_relay

One empty headcrab canister hits an area close to one of the zombie spawns, destroying any props or players nearby.

Possible on: Wave 3, Wave 4


Random Canisters One
random_canister1_relay

Titled "One" instead of "1" for drama, because this is the most dramatic event by far. It begins with a radio transmission (prison_soldier_boomersinbound.wav) being played in the Factory 17 control room, the same place as random_radio1_relay. Shortly afterwards, a single headcrab canister is launched from the city, directly hitting the control room and creating a large hole in the roof, allowing zombies to come in if they can. If there is a barricade here, the canister would most likely be right in the middle of it, killing or injuring nearby survivors. A second canister hits the nearest entrance to the nearest factory, removing/lengthening a potential escape route. Humans will have to think fast if this happens.

Possible on: Wave 4, Wave 5
If triggered a second time, random_water_relay will play instead with extra sounds indicating a mortar strike.


Basement Flood
random_water_relay

The basement in Factory 28 floods following an explosion.

Possible on: Wave 4, Wave 5


Airdrop 2
random_airdrop2_relay

Three flares fall from the sky, followed by a crate containing OP goodies. (if humans can go outside to get this, something is probably wrong)

Possible on: Wave 5