Dictionary
lis'tening post"
1. Mil.a post or position, as in advance of a defensive line, established for the purpose of listening to detect the enemy's movements.
2. any foreign country or city viewed as a source of intelligence about an enemy or rival nation or one that is a potential enemy.
3. any concealed position maintained to obtain information:
The government had listening posts to keep informed of revolutionary activities. Random House Unabridged Dictionary, Copyright © 1997, by Random House, Inc., on Infoplease.