The Geiger counter is a device used to detect radiation. In the Modern Warfare series of games, whenever the player goes into a radioactive area, a clicking noise from the Counter is heard.
It is heard when Cpt. Price and his team of four locate a package in the Call of Duty 4: Modern Warfare level, "Crew Expendable". It was popularized, though, in the Call of Duty 4: Modern Warfare level duo: "All Ghillied Up" and "One Shot, One Kill" where there are radioactive patches scattered around Pripyat, Ukraine and the player must carefully avoid them while completing their objective. It is also used to locate the nuke in Call of Duty: Modern Warfare: Mobilized in the campaign level "Needle in a Haystack".
It is also heard in a few multiplayer maps in Call of Duty: Modern Warfare 2, such as Afghan and Wasteland, near the border of the map, where there is lethal radiation to prevent the player going out of the map and in the Special Ops mission, Hidden.
In Call of Duty: Ghosts, a Federation patrol is seen scanning the area with a Geiger counter in a hazmat suit in the beginning on No Man's Land.
In Call of Duty: Advanced Warfare, Carter asks Joker what his Geiger Counter reads in the mission "Fission" after they leave the nuclear plant.