Drawing bounding box based on optical flow

Based on https://www.mathworks.com/help/vision/ref/opticalflowhs.estimateflow.html I am able to get the optical flow vectors. I want to use this flow information to compute a bounding box over my image, so I took the maximum of the returned matrix to fit them into a bounding box. However no bounding box is being printed.

nFrames = 0;
while (nFrames<100)
    frameRGB = vidDevice();

    % Compute optical flow for frame.
    flow = estimateFlow(opticFlow,rgb2gray(frameRGB));

    maxFlowVx = max(flow.Vx);
    maxFlowVy = max(flow.Vy);
    bbox = [maxFlowVx(1:2), maxFlowVy(1:2)];
    annotatedImage = insertShape(frameRGB,"rectangle",bbox,"LineWidth",8);
    
    imshow(annotatedImage)

    hold on
    plot(flow,'DecimationFactor',[5 5],'ScaleFactor',25)
    hold off

    % Increment frame count
    nFrames = nFrames + 1;
end