This paper describes a system for automatically detecting potential targets (that pop-up or move into view) and to cue the operator to potential threats. Detection of independently moving targets from a moving ground vehicle is challenging due to the strong parallax effects caused by the camera motion close to the 3D structure in the environment. We present a 3D approach for detecting and tracking such independently moving targets with multiple monocular cameras. In our approach, we first recover the camera position and orientation by employing a visual odometry method. Next, using multiple consecutive frames with the estimated camera poses, the structure of the scene at the reference frame is explicitly recovered by a motion stereo approach, and corresponding optical flow fields between the reference frame and other frames are also estimated. Third, an advanced filter is designed by combining second order differences between 3D warping and optical flow warping to distinguish the moving object from parallax regions. We present results of the algorithm on data collected with an eight-camera system mounted on a vehicle under multiple scenarios that include moving and pop-up targets.