UN involvement without a prelude, should there be some sort of formal announcement? Why or why not?
As has already been pointed out, the United Nations can make a decision and take an action without any kind of announcement or other such preparation. Once they vote, they can act. That's generally not the reality, though, because the UN is a massive and slow-moving entity. The truth is that their making the decision is the prelude for their action.
The United Nations does not take unilateral action. By definition, it is impossible. By charter, it is illegal. In something like an emergency, the UN Security Council would be called into emergency session, the issue discussed, proposals made, and votes taken on potential action. Without a majority of the Security Council in support, and all of the Permanent Members (US, Russia, Great Britain, France, China) then action cannot take place.
The vote itself is the formal announcement, the matter of record that makes UN action legal. Aside from this, I am also a little unsure what you mean by either "involvement" or "prelude".
It would be easier to answer this if we knew what sort of UN involvement we are talking about. The more the involvement is "emergency" the less you would need a formal announcement. However, I cannot remember the UN ever doing anything without a formal announcement because they have to vote on everything major.
It is better for the UN to announce what it is doing. That way, it can hopefully get public opinion across the world to support it. By announcing what it is doing and why it is doing it, the UN might be able to make people notice its actions and support them. If it does not make announcements, it will be able to gain less support and put less pressure on whoever it is trying to influence.