Since joining a legion, I've had several planet locations shared to me. Some of them were colonized by another player in my legion but were only partially developed. I was able to build some structures for them. I've done this twice even though no one asked me to. Is there some notification that the other player gets automatically, or does he/she just go to the planet and wonder where the new structures came from?

They will wonder where they came from because no alert is given when you build on their planet. (At least that's from the last time I checked)

I think that would annoy me, unless they are awesome structures

From what I have seen your only allowed to build structures that the recipient has researched the tech for anyways so no awesome structures for free

Yeah, probably related to the fact that if you cap a planet with researchable buildings you haven't unlocked yet, they're auto-demo'ed. What happens though if you cap a planet that has mission-based buildings rather than tech-based (as opposed to random artifacts). Do you get those, or are they demo'ed if you haven't completed that mission yet?

any building (artifact, NPC, mission, etc) that is not normally built with credits and minerals will provide it's bonus to whoever has the planet, regardless of what they have or have not done.

So direct mission rewards would be kept, but mission rewards that resulted in building plans would be demo'ed, unless you also have those plans. Ok, thanks.