Interesting, Anne. Thank you.
And a reminder that Kate McCann's status as arguida left her at liberty to refuse to answer the famous 48 questions. It is almost contradictory that police would have conferred that fifth-amendment like status on her at the same time as wanting her to answer the questions they devised, isn't it?
I am no expert on Portuguese law, but talking to a couple of colleages it seems that Portuguese law diverges somewhat from other european codes.
My understanding (and I am happy to be corrected of course) is that this stems from history. Under the Salazar regime (The Estado Novo) it was effectively a police state.
Someone called in as a witness in a criminal investigation was (and still are) legally bound to co-operate with the police, do not have the right to silence and face legal actions if they lie. They were also not entitled to be legally represented.
With the overturning of the regime, and with Portugal becoming a decomcratic state, the Salazar code was not rewritten, but was modified to provide for rights. The right to remain silent, and to legal representation by either being declared, or declaring onself, an arguido. This provided for rights similar to those under other regimes, but not exactly comparable.
Once an arguido, one will remain one until the end of the investigation.
And you are correct, Sherlock, the root is the latin arguere - which is also the root of Argue in English.