The element ID is "Backpack_Base_FindItems_Button" and it is on the same level as "Backpack_Base_Close_Button".
Example code (Not standard values)
Code:
<PanelFile ID="ID_UISkin_BackpackView6_Template">
<Element ID="BackpackView6_Template" X="100" Y="100" Width="197" Height="126">
.
.
<Element ID="Backpack_Base_Close_Button" X="181" Y="0" Width="16" Height="16"> </Element>
<Element ID="Backpack_Base_FindItems_Button" X="163" Y="-3" Width="16" Height="16"> </Element>
</Element>
</PanelFile>