There's information leakage on planets. You can see what foreign planets are building (social and military) with only "very low" espionage level. Clicking on a planet gives a stardock building "Idle" (should probably read "Unknown"). However, if you open the planet sidebar and tell it to show foreign planets, it lists the active social and military projects.
Easy to reproduce: start a tiny galaxy, 9 enemies. B-line your flagship to be able to see a nearby occupied planet. Pull up the planet list, click foreign. It lists the ship and building being worked on.
