Horde

A horde (also called a wave) is a large amount of awaken enemies being spawned by different events happening in the game. This is the major threat to deal with in GTFO, during the expeditions.

A horde that's spawning will usually emit a deep roar sound, allowing you to locate where the horde is coming from.

The enemies spawned during a horde mostly have a darker shade of skin compared to enemies which are sleeping, however these enemies can also be found sleeping in some expeditions.

Alarm
The alarms will spawn hordes continuously until they are stopped. They are the most common way to spawn the hordes.
 * Alarmed doors will start an alarm when the players will do the security scan sequence in order to open these doors. Some of the alarmed doors are required to open to reach the main objective. In order to stop the alarm, the players need to complete the bioscans by standing in the red areas on the ground.
 * Interacting with a reactor will most likely start an alarm. Depending of the interaction and the objective, the players must complete a designated task, usually commands to enter to the reactor's terminal or bioscans.
 * Interacting with some objective items will trigger an alarm. Some of these alarms can't be stopped and the players must get out of the expedition.
 * The extraction's bioscan can start an alarm in some objectives. These alarms will ring until the players complete the final extraction's bioscan.

Scout's scream
If the players alert a Scout, it will emit a loud scream, spawning an extra horde among the now-awakened enemies around it.