Hi @maple,
the boxes in the left part of the lane indicate a lane set. You can have lanesets nested within each other. Something like this:
.
Also see the respective BPMN XML:
<bpmn:laneSet id="LaneSet_0a3f5ju">
<bpmn:lane id="Lane_020s3j1" name="Department A">
<bpmn:childLaneSet id="LaneSet_14trla8">
<bpmn:lane id="Lane_1e7l5fs" name="Team 2" />
<bpmn:lane id="Lane_0g3wizy" name="Team 1" />
</bpmn:childLaneSet>
</bpmn:lane>
<bpmn:lane id="Lane_0i8u052" name="Department B" />
</bpmn:laneSet>
This means the screenshot you showed shows a Department 1 (which is a lane set) which has an empty lane inside.
Does this help?