Sometimes when I'm approaching an alien-infested area but haven't made contact yet, I'll get a notification from one of my soldiers that they can hear something. This is accompanied by a visual indicator giving me a direction.
Usually, following up means that I will come across hostile forces in the direction where they heard noises.
How does this work exactly? When in the turn will I hear aliens if I'm getting close? How close do I have to be in order to hear hostiles? Is this always accurate? Are there certain enemies who are silent, or who can fake sound so that I walk into the wrong area/a trap?