Soft target is a military term referring to unarmored/undefended targets needing to be destroyed. For example, a soft target would be an automobile, a house, or assembly of people while 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 their schedule or avoids patterns, has solid looking security measures on their home, or other forms of visible readiness would not be a soft target. A soft target for surveillance would have little variance in travel patterns and a visibly low state of situational awareness.