What about a large ship?
I’m no expert, but subs can hide under floating vessels, near the sea bottom, or even blend in with whales.
On the other hand ...
OldMissileer: “That is a sub launched ballistic missile and it looks as if its’ trajectory was taking it toward Kwajalein Atoll where we target our western test launches.”
http://www.freerepublic.com/focus/news/2624382/posts?page=47#47
But if it was a test goof-up, the delay on an official confession is strange.