Killing a soldier: nanomachine warning

So, I was playing in a stealth-ish way through chapter 2, when I realized something: when I killed a PMC soldier, some seconds passed by, when a running comrade comes blazing through from the middle of the forest to see if something is wrong!!

Then, he checks his colleague body and enters a caution state. I've never read about this anywhere. So, apparently, this is the main downside of killing someone if you want to be stealthy.

My questions are: am i getting something wrong? Does this happen through the entire game or only in this particular chapter? What's the explanation for the warning? Nanomachines?