Eye witnesses can be mistaken, hallucinating or lying, radar can be faulty, reading a radar is subject to human error, and conclusions can be jumped to by people all along the chain of information without knowing all the facts. That's a lot of things that should keep us skeptical about these reports.
Take that report with the jet and the radar and the object that apparently dropped from high altitude to sea level then seemingly travelled 60km in a second. If the thing had no visible markings on it, then how do they know that the one 60km away was THE SAME ONE?
What if the military (ours or China's) was testing out some new kind of bouncing bomb, spy drone or missile that could be launched stealthily, possibly even from crazy high altitudes or from space, which then would travel at super-high speeds towards the earth, level off or bounce on water, before disappearing under the water?
Now what if they decided they'd launch two of them for the test, 30 seconds apart from each other, targetted 60km away from each other and the pilots just didn't spot the second one until the first one disappeared? If we assume that there's only one object then "whoah it travelled 60km in a second.... nothing on Earth could do that!" but if there were actually two in the first place then, oh, no big deal, another one 60km away and no unearthly speed is necessary to get it there.