en de visita oficial: why no article?

Hola Pat

Estar de visita oficial follows the same pattern as for example estar de vacaciones (to be on holiday), estar de baja (to be off work on sick leave) , estar de viaje (to be travelling), estar de luna de miel  (to be on honeymoon), etc. In all these cases, de + noun (without any article) forms a fixed idiomatic expression describing someone's temporary status or activity. 

We use it to indicate that someone is temporarily in a particular situation or engaged in an activity.

I hope this clarifies it.

Saludos

Inma