Available in LIB version 2016.3.1128, it will be also available in the 2017 R1 Release.

Available in the 2017 R1 release (2017.1.117).

Sample:
<telerik:RadGridView ShowSearchPanel="true" ShowSearchPanelCloseButton="false"
or
<telerik:RadGridView ShowSearchPanel="true" ShowSearchPanelCloseButton="Hidden"

Workaround: reused the cell element like this:
CreateCellElement(Telerik.Windows.Controls.GridView.GridViewCell cell, object dataItem)
{
    var button = cell.Content as Button;
    . . .
 
    return button;
}

Added the CanUserSearchInHiddenColumns property which indicates whether users can search for content in hidden columns. The property is available with our 2017.3.1127 internal build.