Mouse is an achievement/trophy in Metro: Last Light awarded for completing Echoes without being caught by the pack of watchmen. However, the achievement/trophy can be unlocked after you find the Duplet in a side passage, and are ambushed by a watchman. After leaving this area, follow Pavel up the stairs until he asks you to stop. A pack of watchmen will run across the landscape. As long as you keep still, they will not see you, you will gain moral points and unlock the achievement/trophy. However, if you move too much or attack them, Pavel will complain, you will lose moral points and the pack will attack. Below is a video guide if you are having trouble.