'Nuclear launch detected,' is a shortened version of the passive sentence 'A nuclear launch has been detected.' In this sentence the verb 'to detect,' is still transitive but the word order has been changed compared to a sentence in the active voice... here the object of the sentence is at the start - it is the nuclear launch. 'We have detected a nuclear launch,' subject = we, verb = detect, object = nuclear launch = sentence is in the active voice. 'A nuclear launch has been detected,' object = nuclear launch, verb= detect = sentence is in the passive voice. There is no obvious subject (the subject is simply implied or suggested ...the launch must have been detected by somebody and that somebody is the implied subject). In a passive sentence there is always a direct object so that the verb must be transitive.