Dumpster diving is the act of searching the contents of trash containers for something of value. In a pen test, dumpster diving can help you claim certain documents that contain sensitive information relevant to the organization. For example, in the first few weeks of the year, people often discard calendars from the previous year. Many people write their passwords down on their calendars so they don't need to remember them. In addition to personal documents, organizations sometimes improperly dispose of official documents in hard copy, like past quarterly financial reports or product proposal drafts. These can give you an insight into the target's business operations. You may even be able to piece together shredded documents with enough time and patience.
In addition to documents, organizations also improperly dispose of storage drives and even whole computers. They may have failed to wipe the data from these devices, enabling you to recover their contents and possibly find something of value.
Like fence jumping, dumpster diving will likely draw suspicion if you're seen. Still, dumpsters are usually placed out of view and away from where people work. Dumpsters may also be conveniently accessible outside of restricted areas, so that external sanitation personnel can pick up the trash without needing to go through a security checkpoint. In other words, they may be exposed to the public and require little effort to access.