One of the things several countries have been working on (US, China and Russia at the very least) for the past few decades are actually remote spacecraft that can rendezvous with other satellites, either to observe, intercept communications or potentially sabotage. One recently launched craft was actually observed by amateurs to move close to the ISS (within 4 miles at one point, EXTREMELY close by space standards) during one of their docking/undocking procedures. Thought by some to be a test of its capabilities.
If there is a "stealth" satellite, there's a decent probability it'd have a mission like this rather than your typical "take pictures of the ground" sat that while "secret" nobody really tries to hide all that hard.
There was also speculation about the specific orbit it was put in, ~50 degree inclination, which specifically covers all the oceans other than the far north arctic. But that doesn't really prove anything because that also covers basically all relevant land as well.
Given the news though, and the relevant statements by government officials, I'd say it's more likely that it did fail in some manner. It might still be in orbit, perhaps temporarily. It might still be functional in some regard, perhaps temporarily. But I'd say likelihood of it either already, or soon being in the ocean is pretty high.