Consider the following:

Map<String, String> map = [foo:'bar']
assert map['foo'].toUpperCase() == 'BAR'

When executed, the runtime selects DefaultGroovyMethods#getAt(Object,String): Object for the subscript operation. However, the editor is inferring DefaultGroovyMethods#getAt(Map,Object): String and so "toUpperCase" is highlighted as an instance method of String. Since the return type is Object the method should be highlighted as unknown (underlined).

NOTE: There are separate issues filed to handle the oddities that result from having getAt(Object,String) and putAt(Object,String,Object) extension methods.