Also reminds me of something I read just yesterday:

The urban guerrilla, living in the midst of the population and moving about among them, must be attentive to all types of conversations and human relations, learning how to disguise his interest with great skill and judgement.

In places where people work, study, and live, it is easy to collect all kinds of information on payments, business, plans of all kinds, points of view, opinions, people’s state of mind, trips, interior layout of buildings, offices and rooms, operations centers, etc.