Questions from a newcomer (mild spoilers)

First of all, I've been searching around, trying to find ways to play offline, which does not seem possible. It does seem you can sort of stay single player though, to some extent. I really, really dislike other players in my games and I would much prefer playing offline and on my own, but given that is not possible, I am trying to find the next-best solution. For now, I have disabled every box in the "Connection" tab of the game options and I have made my profile private, which has helped some. I still occasionally see other players, mostly in town, but also (rarely) out in the world. Is there really no way to completely hide other players? I really feel that seeing other players ruin my immersion in the game, so I hope I can get rid of them completely.

Another question (and this is where the slight spoiler comes in): I play as a rogue and did that first quest for rogue specialisation skills. I got a pair of boots and a dagger. When I got the boots, I got a notification about that they had some sort of stealth ability that I should equip in the "Attributes" screen. I assumed it was the skill tree, but I could see nothing new on that screen. Is that skill just automatically equipped or do I need to go somewhere else to equip it?

I also occasionally complete some challenge, where I feel the wording leads me to believe I need to go claim the reward for completing that challenge somewhere, but I can see no claimable rewards in the challenge screen. So maybe I just automatically get the rewards and I haven't noticed?

Final question (for now): I recently noticed that next to the bar with my equipped skills (at the lower left of the screen), there are three yellow, gem-like triangles that seem to sparkle, as if there is something that needs my attention, but I can't figure out what it is. Since it is part of the GUI, I assume it is trying to tell me *something*, but I don't know what.