It rather suggests they're going to someone or something devoid of a definite article (going to Flore). "Aller au Flore" means "Going to the Flore", with the im

It rather suggests they're going to someone or something devoid of a definite article (going to Flore). "Aller au Flore" means "Going to the Flore", with the implied idea "going to the café de Flore" (this is the normal way of saying it for any café, restaurant...). So it indicates, for the speaker, that (the) Flore is more than a café.