You can see this issue in this example: http://docs.sencha.com/ext-js/4-0/#!/example/grid-filtering/grid-filter-local.html
Try to filter "Company" column, and you will not see any icon near text field.
I got some source code from RangeMenu (my idea was the principle of working NumberFilter) to fix this issue (please replace all "Ext4" to "Ext"):

PHP Code:
init : function (config) {
        
Ext4.applyIf(config, {
            
enableKeyEvents true,
            
hideLabel       false,
            
fieldLabel      this.getIconTpl().apply({
                
cls  this.iconCls || 'no-icon',
                
text this.emptyText || '',
                
src  Ext4.BLANK_IMAGE_URL
            
}),
            
labelSeparator  '',
            
labelWidth      29,
            
listeners       : {
                
scope this,
                
keyup this.onInputKeyUp,
                
el    : {
                    
click : function (e) {
                        
e.stopPropagation();
                    }
                }
            }
        });

        
this.inputItem Ext4.create('Ext4.form.field.Text'config);
        
this.menu.add(this.inputItem);
        
this.updateTask Ext4.create('Ext4.util.DelayedTask'this.fireUpdatethis);
    },

    
/**
     * Возвращает шаблон иконки
     * @return {Ext4.XTemplate}
     */
    
getIconTpl : function () {
        return 
Ext4.create('Ext4.XTemplate',
            
'<img src="{src}" alt="{text}" class="' Ext4.baseCSSPrefix 'menu-item-icon ux-rangemenu-icon {cls}" />'
        
);
    },