I've noticed occasionally enemies show up at your outposts, but the times it has happened to me, I was there and killed them pretty quickly. Most recently, I built an outpost on a fairly high leveled planet, and immediately saw a bunch of spacers show up, but between me, my follower, and local wildlife, they died before doing any damage.
I haven't noticed any attacks occurring while I have been away however. I haven't seen any notifications saying they were being attacked, nor have I noticed any damage to my outposts. Can my outposts be attacked while I am away, or are they just events that occur while you are there?