Soft target is a military term referring to unarmored or undefended targets intended to be destroyed. For example, a soft target would be an automobile, a house, or assembly of people. By contrast, a hard target could be a main battle tank or a well defended installation.
This can also be applied to apparent readiness. A person who varies his or her schedule or avoids patterns, has strong security measures for his or her home, or other forms of visible readiness would not be a soft target. In surveillance, a soft target would have little variance in travel patterns and a visibly low state of situation awareness.