Modify your Datatrigger like this:
<ControlTemplate.Triggers>
<DataTrigger Binding="{Binding ElementName=lbl, Path=Content}" Value="UNKNOWN">
<Setter TargetName="exp" Property="IsExpanded" Value="False" />
</DataTrigger>
</ControlTemplate.Triggers>
and name the expander:
<Expander x:Name="exp" IsExpanded="True" FlowDirection="LeftToRight">
...
Note: the expander must be defined before the trigger:
<ControlTemplate TargetType="{x:Type GroupItem}">
<Expander ...
<ControlTemplate.Triggers> ...
</ControlTemplate>