I do not understand the motivation of announcing this. First, it was clearly detected, however they claim it was not early enough. Regardless, it sends a messa

I do not understand the motivation of announcing this. First, it was clearly detected, however they claim it was not early enough. Regardless, it sends a message to Russia that they know of their activities. Second, I doubt the veracity of this claim - what is the benefit, besides political, of this announcement? My hypothesis is that a sonar array heard the sub entering the gulf, and it was treated as a military exercise to try detecting it using passive measures - i.e. to assess our capabilities of detecting the new type of submarine, or to study it by having it trailed. If the Pentagon felt that there was actually a threat, I think that they would conduct some unannounced "war game" with active sonar in the gulf to locate the sub more quickly. Regardless, this sends the message that we are not completely oblivious, like saying "We don't know whether Putin put creamer in the coffee he had this morning at 7:38 am in this specific presidential bunker."