The planetary invasion tutorial screen pulled a houdini on me. It showed up while it was processing the next turn, and when the next turn started it had vanished on me.

...I don't suppose there's any way to get it back, is there?

It might've had something to do with finding the Vodyani via probe (found their sphere, but hadn't spotted the system yet). At least, that's my guess, since it happened around that time (and an invasion tutorial would make sense for your first encounter with another empire). Alternately, there's a minor faction in a strategic system two jumps away from my homeworld (but they're pacifists). I doubt it was this, but you never know.