Alarm Systems are in many ways similar to traps, and are just as frequent in the metro. Many routes will have traps that will alert any nearby enemies of Artyom's presence. Although they are not deadly like tripwires, this does not make them any less of a threat. Many alarm systems in the metro are simple yet effective improvisations ranging from broken glass on the ground to cans hanging from a ceiling by a line of rope. Triggering the alarm is usually bad news, especially if the player is attempting to pass the area without killing.
The Alarm system is very common in Metro 2033, in Metro: Last Light however, the Alarm system is less likely to be encountered, as the stealth system simplfied at player's favor, and the NPCs are easier to sneak up.
These alarms are quite simple and extremely easy for people with untrained eyes to over look. Broken glass makes a loud crunching sound when walked over and will alert most enemies in the vicinity to that area. Bandits and Nazis make use of this alarm system frequently. Glass is first encountered as an obstacle when traveling with Bourbon on Lost Tunnel.
Another simple yet highly effective alarm system used extensively by many factions throughout the metro system. Once touched, the cans rattle causing a fair amount of noise and will alert anyone who hears it to that location, or they will keep constant attention on the location of where the cans were disturbed. This alarm system is seen in every bandit controlled location and is also used by the defenders of many stations. This is most notable at Cursed station and Hole station as many hanging can alarms are seen in the area to alert the defenders of any approaching mutants.
Not as common as the two other alarms systems, but still just as dangerous. The bell is by far the loudest of any alarm system and will put all enemies in the area on alert when sounded. On levels such as Dry for instance, once the alarm bell is rung, the whole station will be on alert making it very difficult to progress through the level. Sometimes bells are attached to trip wires and will sound once tripped, this kind of tripwire cannot be disarmed. More commonly, enemy NPCs will ring them after spotting the player.
Not so much of an intended alarm system but more of an obstacle that the player will have to be very wary of. Bones, and other human remains that work as alarms are only seen in the Depository. They are seen close to the areas where black librarians are sleeping, and should be avoided at all costs (as black librarians are some of the strongest and most dangerous of mutants). Once trodden on (or accidentally touched), the bones will make a very loud snapping sound, instantly waking a sleeping librarian and causing it to attack the player.