Answer:
Security
Explanation:
In army related activity, observer refers to any thing that is used to collect information for the intelligence team.
In the past, the army could only rely on human labors to collect such information. But in recent years, the development of small high quality cameras and drone technology revolutionized the way army could collect their information.
Enemies could place a small observer and camouflage it without we even realizing it and it could leak the entire operation. Making them become well prepared to counter it.
An example of a body movement to show positive could be a thumbs up, a negative could be a thumbs down. With facial expressions you can smile, or frown.
