The ZED SDK provides the left and right rectified images of the scene and computes the 3D position of each pixel. When an object is too far or too small, the SDK will detect that the estimation isn't accurate and will leave its pixels as not estimated. We recommend using OpenCV with color segmentation or shape detection.
You can use ArUco markers. An ArUco marker is a synthetic square marker composed of a wide black border and an inner binary matrix that determines its identifier.
Stick the ArUco stickers on the objects you want to track, and using the ZED SDK, you'd be able to detect the objects accurately.