Informer

A grineer Informer.

Informers are Grineer spy drones introduced in the Spy Drone Event starting on July 5th, 2013. They appear as miniature weather balloon-esque sentry drones that float in place on random points of the map, or drift around slowly. Informers cannot attack, or even move aside from hovering slightly in the same space. Informers are equipped with Orokin cloaking tech, which they will activate upon being damaged in order to escape, meaning they must be killed with the first shot. The weak spot on the Informers is the orange sac-like region.

Top view of an Informer

During the duration of the event, Informers will be found on any Grineer missions that are level 15 or above. Killing 20 will qualify the player for receiving the Snipetron Vandal as a reward after the event ends. They can be often found on Grineer systems above level 15 (Earth, Sedna, Ceres) in Grineer missions, Especially on Cambria (Earth).

Trivia

• Informers have the special ability of disrupting the player's mini-map when in its proximity, which is also an effective way of knowing when one is nearby. They can also negate Enemy Radar at a radius slightly larger than its mini-map disruption radius.
• Informers will disappear upon receiving damage from Grineer units as well.
• Informers will respond to your movements and can "see" you, turning towards your direction while staying in the same postition.
• Given a long enough radar range, it is possible to detect an Informer with Enemy Radar by staying out of its range with altitude.
• Informers spawn on ground level, before floating up to its proper height.
• Banshee's Sonar can spot them even when your map is almost completely disrupted by them.

Notes

• Since the Informer will escape if the first hit doesn't kill them, high-power accurate weapons such as the Paris, Lex, Glaive, Vulkar, and Latron should be used if you are hunting them. Also Kunais and Despairs with a few damage mods are able to destroy them.
• However, weapons such as shotguns or burst pistols like the Kraken can also easily destroy them. Using a shotgun is not a hinderence as you can wall jump then while you're sliding down the wall fire.
• Finally, high-power splash damage weapons (such as the Ogris) are also effective for taking down clusters of Informers.

• Sentinel fire and abilities will cause it to disappear, unless the damage is high enough (like Dethcube's Vaporize), however they will not target it directly.
• Informers are plentiful on Cambria (Earth), yielding 20+ if the map is searched thoroughly.
• Informers can be found on Kappa(Sedna) and have a 100% chance of spawning.
• Informers can be found in number from 1 to 5,6 on Sycarox (Uranus)
• Informers can be found in great numbers on Prospero (Uranus).  If you clear the map, complete the objective, and reclear the map, you can take down all 20 in one run.
• Setebos (Uranus) shows similar numbers to Prospero but is a sabotage mission instead of a raid mission and can contain much larger rooms.
• Kiste (Ceres) can also sometimes yield high numbers, ranging from just a few to 20+. Solo play tends to yield better results, given the player is equipped to handle the large number of enemies found in mobile defense missions. 
• Every Frame Power with an area of effect (such as Rhino Stomp, Avalanche, Overload, Mag Crush, Radial Javelin, Slash Dash, etc.) can destroy the Informers. Ash's Shuriken power will destroy Informers of any level. (Other Powers need confirmation)
• Despite the emphasis on destroying Informers 'as quickly as possible', this should rarely present a challenge to any Tenno unless they are using an exceptionally weak weapon; the Informers have very thin armour and weak shielding, to the point that an Informer on Cambria can be destroyed by a single hit from Frost's Freeze ability, if it's at maximum rank.
• Doing solo runs increase the number of Informers to spawn compared to doing it with a squad or another player.