The humans have discovered something. All of their diplomats, their scientists, their military highest, they're all excited and a little nervous about something. Our people are still free to come and go in their space, but everyone is being heavily scrutinized. While the average citizen may not notice anything, intelligence agents have noted a massive increase in surveillance drones, satellites, and sensors of all sorts monitoring any non-human. See file xhdlpj552-21 for more information. One possibility is that they are planning to invade Union space, though this is unlikely. It's more likely that there's another sapient species past their territory that they've discovered, or that they've invented something with troubling implications.
In the event of the latter, it seems unlikely that they will share such technology until such a time that they are secure in their supremacy of it since they remain fairly nationalistic despite our best efforts to promote infighting and factionalism. Hopefully, those efforts will be more effective over time. There are a few paths we can tread. We can ignore this. We can approach some of the more mercenary humans and see if we can hire them to figure out what's happening. We can send spies with the best espionage technology we have.
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To preempt some people's suggestions, invading human space is not a wise choice. While we have an immense advantage in sheer number of people, the Terran Confederacy definitely has the advantage in augmentation, cybersecurity, big guns, and a much smaller area to defend.
The only information we really have is an intercepted message promising "Project Reality Diner" an unlimited budget. No matter what we choose to do, something big is coming.