I have not found any human players yet (as you know) but today I discovered the first aliens. It's the ligh colored greens (whatever their name is, Kyrat or such) but unfortunately they occupy the same atmosphere rocks as I do.
I have succeeded in colonizing the juicy system I mentioned a few days ago, and to stabilise my mineral production, though I fear it delayed other efforts. I had to spend 3 turns researching minerals II so I could upgrade my home planet's miners.
I would love to explore further, but am a bit paranoid. On my frontier are two nebulae, and I am afraid that if I send my small exploratory force through it, a human player will slip by me and attack my recently established colonies. Oh well, such are the challenges of SEIV