All items which are tagged as camper-sensitive, which by default include the rocket launcher, grenade launcher, quad damage, red armor, and lightning gun, look around themselves about every 8 seconds. They have two detection ranges: a long range that includes only the item’s line-of-sight, and a shorter range that can detect players even without LOS. If a player is detected for two consecutive think times, they are issued a warning of the form “**CAMPER WARNING FROM (item name)”, their screen flashes and a sound is played.
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(Source: gamers.org)