It's a planetary roamer, then. Not a huge risk given how long it probably takes to buy, I'll continue to pay attention, but I doubt it's anything to worry about now that my fleet can clear homeworld adjacent systems without the time and resources that MKIVs required me to sink into them. Plenty of opportunity to clear the fleet and posts ahead of time so I can group up on the starships when they actually appear. And this was only a MK1 Heavy Bomber, not the MK4 and MK2 I noticed in a previous game (and that might have been a response to a dyson).
Attached save, old version, Planet Mbzonious, near ai command, adjacent to my homeworld, MKIV flagship, MK2 Zenith Starship, Planetary Roamers, 15 minutes.
Edit again: Reinforce logs from around that time don't show any spire starship purchases or flagship purchases, indicating it might have been a side effect of the strength 10 dyson.