If the subject is not given directly but it is response to a question asked to usted.
Example of question to usted : ¿Le apetece a usted tomar algo?
Answer : Me apetece eso.
Explanation : The subject of this sentence is "Eso", a demonstrative pronoun, which is usually used pointing to something. But it makes it unnecessary to specify the subject. "Me" is a indirect object personal pronoun, which means "a mí".
Translation : Do you fancy drink something?
That fancies to me.
Hope this helps!
