Gazebo collide without contact

collide_without_contact.world

<sdf version='1.4'>
  <world name='default'>
    <!-- include sun and ground plane -->
    <include><uri>model://sun</uri></include>
    <include><uri>model://ground_plane</uri></include>

    <model name="sensor_box">
      <static>true</static>
      <link name="link">
        <pose>0 0 0.251  0 0 0</pose>
        <collision name="collision">
          <max_contacts>1</max_contacts>
          <geometry>
            <box>
              <size>4 4 0.5</size>
            </box>
          </geometry>
        </collision>
        <visual name="visual">
          <geometry>
            <box>
              <size>4 4 0.5</size>
            </box>
          </geometry>
          <material><script>
            <uri>file://media/materials/scripts/gazebo.material</uri>
            <name>Gazebo/Grey</name>
          </script></material>
        </visual>
        <sensor name="box_contact" type="contact">
          <always_on>true</always_on>
          <update_rate>1000</update_rate>
          <contact>
            <collision>collision</collision>
          </contact>
        </sensor>
      </link>
    </model>
    <model name='contact_box'>
      <pose>0.0 -1.0 1.5  0 0 0</pose>
      <allow_auto_disable>false</allow_auto_disable>
      <link name='link'>
        <collision name='collision'>
          <geometry>
            <box>
              <size>1.0 1.0 1.0</size>
            </box>
          </geometry>
          <surface>
            <contact>
              <collide_without_contact>false</collide_without_contact>
            </contact>
          </surface>
        </collision>
        <visual name='visual'>
          <geometry>
            <box>
              <size>1.0 1.0 1.0</size>
            </box>
          </geometry>
          <material><script>
            <uri>file://media/materials/scripts/gazebo.material</uri>
            <name>Gazebo/Grey</name>
          </script></material>
        </visual>
      </link>
    </model>
    <model name='collide_box'>
      <pose>0.0 1.0 1.5  0 0 0</pose>
      <allow_auto_disable>false</allow_auto_disable>
      <link name='link'>
        <collision name='collision'>
          <geometry>
            <box>
              <size>1.0 1.0 1.0</size>
            </box>
          </geometry>
          <surface>
            <contact>
              <collide_without_contact>true</collide_without_contact>
            </contact>
          </surface>
        </collision>
        <visual name='visual'>
          <geometry>
            <box>
              <size>1.0 1.0 1.0</size>
            </box>
          </geometry>
          <material><script>
            <uri>file://media/materials/scripts/gazebo.material</uri>
            <name>Gazebo/Grey</name>
          </script></material>
        </visual>
      </link>
    </model>
    <gui fullscreen='0'>
      <camera name='user_camera'>
        <pose>6.5 0.0 1.5  0.0 0.0 -3.14159</pose>
        <view_controller>orbit</view_controller>
      </camera>
    </gui>
  </world>
</sdf>