I have a LayoutPanel with a LayoutDocumentPane and a LayoutAnchorablePane. If I close all the initial LayoutDocuments inside of the LayoutDocumentPane before progmatically adding the first LayoutAnchorable to the LayoutAnchorablePane, the LayoutAnchorablePane is gone. Here is my layout:
 
<ad:LayoutPanel Orientation="Vertical">

<ad:LayoutPanel Orientation="Horizontal">
    <ad:LayoutDocumentPane>
          <ad:LayoutDocument Title="Left Doc"></ad:LayoutDocument>
    </ad:LayoutDocumentPane>
</ad:LayoutPanel>
<ad:LayoutAnchorablePaneGroup DockMinHeight="200" DockHeight="200">
    <ad:LayoutAnchorablePane Name="BottomAnchorablePane">
    </ad:LayoutAnchorablePane>
</ad:LayoutAnchorablePaneGroup>

</ad:LayoutPanel>
 
 
Here is the code to find the anchorable pane:
 
LayoutAnchorablePane pane = layoutRoot.Descendents().OfType<LayoutAnchorablePane>().FirstOrDefault(p => p.Name == "BottomAnchorablePane");