Try something like this:

// Get the trackable:
const QCAR::Trackable* trackable = state.getActiveTrackable(tIdx);
QCAR::Matrix44F modelViewMatrix =
QCAR::Tool::convertPose2GLMatrix(trackable->getPose());

// take the inverse of the modelview matrix to find the camera orientation in relation to a target at the origin
QCAR::Matrix44F inverseModelView = SampleMath::Matrix44FTranspose(SampleMath::Matrix44FInverse(modelViewMatrix));

// pull the camera position from this matrix
QCAR::Vec3F cameraPosition(inverseModelView.data[12], inverseModelView.data[13], inverseModelView.data[14]);

// project the vector to the X/Y plane
// reverse to get a vector pointing to the target
QCAR::Vec3F projectedDirectionToTarget(-cameraPosition.data[0], -cameraPosition.data[1], 0);

// normalize the projected vector
projectedDirectionToTarget = SampleMath::Vec3FNormalize(projectedDirectionToTarget);

// choose Y up as the forward-facing vector for the marker
QCAR::Vec3F markerForward(0.0f, 1.0f, 0.0f);

// find the dot and cross products of the vectors
float dot = SampleMath::Vec3FDot(projectedDirectionToTarget, markerForward);
QCAR::Vec3F cross = SampleMath::Vec3FCross(projectedDirectionToTarget, markerForward);

// the angle between two vectors is the acos of the dot product
float angle = acos(dot);

// acos returns an angle in radians, convert to degrees
angle *= 180.0f / M_PI;

// our angle is now between 0 and 180, use the cross product to make it between 0 and 360
if (cross.data[2] > 0)
    angle = 360.0f - angle;

LOG("angle: %f", angle);

This should get you an angle between 0 and 360 as you rotate the marker in front of the camera (or as you walk around the marker).

- Kim