Huh, we didn't mind showing that?
The US Navy maintains that the USS Michigan, a submarine known for carrying special-ops teams, stopped in the South Korean city of Busan for a "routine port visit," but pictures of the event suggest a more clandestine purpose that may involve US Navy SEALs.
On top of the Michigan as it arrived in Busan appeared to be two silos for SEAL Delivery Vehicles, the tiny submarines used to transport US Navy SEALs and their equipment for their most covert missions deep in enemy territory.
If we didn't want that seen, it wouldn't have been seen.
[And I'm reasonably sure that the part quoted saying the submarine is known for carrying special-ops teams was intended to say the class is known for carrying cruise missiles.]
Remember, President Trump's statements and Tweets along with displays of military power might serve to get North Korea to back down or convince China that they'd best deal with North Korea lest America do it in ways that China can't influence to benefit them.
Indeed, China may be the intended audience:
David Petraeus interprets all of the "rhetoric" about the North Korea nuclear threat as being directed at China, rather than Pyongyang, to convince Beijing to get tough in negotiations as it could have the ability to strike a U.S. city "on this president's watch."
Which makes sense because I suspect that we've given China a warning to do something about North Korea before they have nuclear missiles or we will do something.
So the rhetoric reinforces whatever China sees America and our allies doing quietly to prepare for the day when we can do something if China hasn't acted.
Kim Jong-Un may need to double the guards because of the threat from China rather than America and our allies.