How to reproduce: add a MapCallout to a layer and then try to hit test it in the MouseDown event of the control. The element is not always returned even though it contains the mouse location:

private void RadMap1_MouseDown(object sender, MouseEventArgs e)
{
    PointL point = new PointL(e.X - this.radMap1.MapElement.PanOffset.Width, e.Y - this.radMap1.MapElement.PanOffset.Height);
    MapCallout callout2 = this.radMap1.Layers.HitTest(point) as MapCallout;
    if (callout2 != null)
    {
        RadMessageBox.Show("MapCallout!");
    }
}