We are also assuming they even care to travel in space to visit other cultures. Maybe they don't care, maybe they are afraid of exposing their own civilization, maybe they can observe without being observed. Maybe they are already here, guiding humanity (to succeed, or to fail).

Ok, haven't read it but I will give it a shot - I did notice we are off the timeline just a little bit - interstellar travel was suppose to be possible in 2008 in the book

Possible Explanations no particular order:
1. Natural Phenomenon (Unlikely though think Lenticular Clouds)
2. Fraud/Joker (Hard to pull off - could be U.S. government fraud, as a deterrent or IO campaign)
3. Element of U.S. government (why is U.S. Government revealing now? same as above, deterrent or IO campaign?)
4. Foreign government (plausible - might only take a single break-through in physics)
5. Other advanced "secret" society (Say, a group that kept on learning instead of stalling through the dark ages, now several hundred years past us in learning - must be large enough to sustain themselves but small enough to keep hidden - also hard)
6. Parallel Universe visits
7. Time travelers (hardest for me to believe)
8. Aliens - I believe the conditions to sustain advanced life long enough to achieve space travel are rare, but as been pointed out numerous times, the universe is big. Their intentions? Could go either way. Maybe they are already dictating how our society should progress, including how to reveal their presence...
Others?

Yes, I agree and of course if there are multiple they didn't all arrive at the same time and probably don't all have the same capabilities, so who got here first/has the stronger position - benevolent, indifferent, controlling or not helpful/antagonistic and what does that mean for us?