... 
 <controls:RadAutoCompleteBoxFilter x:Key="RadAutoCompleteBoxFilter" />
...
     <telerik:RadAutoCompleteBox Grid.Row="0"
                      x:Name="CaseAutoCompleteBox"
                      WatermarkContent="{x:Static rs:ResourceText.Start_Typing}"
                      ItemsSource="{Binding Suggestions}"
                      SelectedItem="{Binding SelectedItem, Mode=TwoWay}"
                      TextSearchMode="Contains"
                      TextSearchPath="Name"
                      DisplayMemberPath="Name"
                      AutoCompleteMode="SuggestAppend"
                      DropDownItemTemplate="{StaticResource CasesSearchAutoComplete}"
                      SelectionMode="Single"
                      SelectionChanged="CaseAutoCompleteBoxSelectionChanged"
                      PreviewMouseLeftButtonDown="OnPreviewMouseLeftButtonDown"
                      GotFocus="CaseAutoCompleteBox_OnGotFocus"
                      FilteringBehavior="{StaticResource RadAutoCompleteBoxFilter}"
                    
                      />

 public partial class AddMediaToCase 
 {
     public AddMediaToCase()
     {
         InitializeComponent();
         RadWindowInteropHelper.SetShowInTaskbar(this, true);
     }

     private void CaseAutoCompleteBoxSelectionChanged(object sender, SelectionChangedEventArgs e)
     {
         var autoCompleteBox = sender as RadAutoCompleteBox;
         CaseInformationStackPanel.Visibility = autoCompleteBox.SelectedItem != null ? Visibility.Visible : Visibility.Collapsed;
     }

     private void OnPreviewMouseLeftButtonDown(object sender, MouseButtonEventArgs e)
     {

         if (string.IsNullOrWhiteSpace(CaseAutoCompleteBox.SearchText))
         {
             CaseAutoCompleteBox.Populate(" ");
             
         }


     }

     private void CaseAutoCompleteBox_OnGotFocus(object sender, RoutedEventArgs e)
     {
         if (string.IsNullOrWhiteSpace(CaseAutoCompleteBox.SearchText))
         {
             
             CaseAutoCompleteBox.Populate(" ");
         }

     }
 }

  public class RadAutoCompleteBoxFilter : FilteringBehavior
  {
      public override IEnumerable<object> FindMatchingItems(string searchText, IList items,
          IEnumerable<object> escapedItems, string textSearchPath,
          TextSearchMode textSearchMode)
      {
          var result =
              base.FindMatchingItems(searchText, items, escapedItems, textSearchPath, textSearchMode) as
                  IEnumerable<object>;

          if (string.IsNullOrEmpty(searchText) || !result.Any())
          {
              return ((IEnumerable<object>)items).Where(x => !escapedItems.Contains(x));
          }

          return result;
      }
  }