The last US "test" was in 2008 to destroy a defunct spy satellite. Spy satellites have very low orbits (around 100km) and an overwhelming majority of the debris from that test reentered the atmosphere and burned up within weeks or months. Previous ASAT tests were done in the 80s and were also all at low altitude where debris burns up within a few months. The Chinese ASAT test was at around 1000km and much of that debris will be there for tens or hundreds of years. It basically undid all the work the international community had done on reducing debris over the past decade (http://www.scientificamerican.com/media/multimedia/0212-spac...).

It is possible, but highly unlikely that the US does ASAT tests in secret. It's unlikely mainly because any unannounced rocket launch looks suspiciously like an ICBM (and is easily detectable), so it is in everyone's best interest to announce launches ahead of time. If the US has done secret ASAT tests, they would continue to do them at lower orbits out of concern for debris.

The US tracks all objects in low Earth orbit that are about 10cm or greater in diameter using a large network of radar stations. The Russians have similar capabilities, though not on the same scale. As a result, they actually can tell where most of the debris came from. However in this article it was not clear if it was just conjecture or actual tracking data from the Chinese ASAT debris that led them to that conclusion.

It's kind of hard to blow up a satellite secretly (as in, no one noticed).

The US performed its anti-satellite tests against targets in low Earth orbit, so that the orbits of the debris decayed quickly. That was not the case for the Chinese test, which was in a relatively high orbit and generated a huge amount of long-lived debris.

More so, the idea of "secret" anti-satellite tests is a bit rich, satellites are tracked by RADAR from multiple countries, you can't hide a satellite blowing up.