How does "hearing" work?

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?

Solution 1:

When aliens are moving around they tend to make noise. An alien who moves as a result of you approaching it, but which you cannot see, will give you a directional indicator.

Whenever you first approach an alien it will get an automatic movement turn (even during your turn). They usually will seek cover, although I've noticed the Cyber Discs are particularly aggressive.

Solution 2:

The sound indicator is a sort of help tool to find enemies without searching the entire map. Moving enemies AND enemy spawn points (enemies you havent activated by finding them yet) can give off sound depending on how close your squad member(s) is.

One important thing to note (at least this is what has happened to me) is that the sound indicator gives a very good directional indicator for where the enemy is located IN RELATION TO THE SOLDIER THAT HEARS IT. Also, the indicator shows lateral direction, but does not indicate distance or altitude. The sound can look like it comes from far away, even if the enemy is few squares away, but in an unopened room. The sound indicator may also show sound that seems like it comes from outside a building, when the enemy is on the roof, but in the direction the sound indicator shows.