Article 5 is rather more detailed than that and doesn't mention the word "war" at all. Rather it uses the word "attack"
Actually, the phrase you're looking for, which is very contentious in a hybrid scenario (particularly involving cyber) is '
armed attack'.
So, in the case of your mystery attack by an unseen enemy, the state that was attacked could invoke Article 4 on the grounds that its security had been threatened. Thereafter, it would be for member nations to decide what action should or could be taken. That may be anything from a stiffly worded rebuke to offering security assistance to armed deployments. Every situation is different and it is for NATO to decide how to handle each one accordingly.
However, as CM says obliquely, what constitutes an Article 5 precedent? Answer: whatever 28 nations agree consensus on in the NAC.