At the beginnin of Chapter 2 I walk through Henselt's camp and talk to him. Sile is standing to the right of his tent, Dethmold a little further away to the left.

However, if at any point after entering the upper level of Henselt's camp, I save and reload the game there, Dethmold and Sile are gone. I have a savegame where I am looking directly at Sile - she appears in the savegame icon in the load menu - but when I load the game the spot where she was standing is empty.

Dethmold also disappears, but this is apparently "fixed" by his triggered appearance after leaving Hanselt's tent after the initial conversation with Henselt ( and the Nilfgardian ambassador before that). Sile is gone, though.

As for Henselt, he was still there after my first talk with him but laler in the game he wasn't in his tent any more. I have been playing in chapter 2 for about 2 days now and never seen him anywhere (except for his appearance during the tournament quest)

Am I missing something here - are they somewhere else in the camp or is this definitely a bug?

Have you tried meditating for 24 hours? Sile certainly doesn't stay at his tent and is almost always found in her own tent. Henselt tends to be either in his tent or outside hanging out with his knights.

Which tent is Sile's? The one right nex to Henselt's?

I am not seeing her in any of the tents on the upper level, though.

No, she's on the other end of the royal quarter as I like to think of it. The part of the camp cordoned off from the rest of the camp. Granted, there are only three tents that have actual openings in them up there (not counting the pavillion from which Hensalt watches the gladiator pit), so it's not like you've got many choices to be looking in.

So, you're in the beginning of Act 2, have meditated at least 24 hours, and she doesn't show up even in her tent? If so, then you're probably bugged, since I never ran into a problem finding any of them in their tents. I can't even remember her being involved in any real quests to speak of so you could probably just not worry about her and only get worried if she doesn't show up during a scripted event late in the Act. ;p

If you can't find King Henselt, well, that becomes a little more problematic as he is definitely involved in a few quests.

Thanks for the advice - it turns out that meditating DID help. Doesn't even have to be 24 hours. A couple are enough.

Suddenly there is Henselt with some of his advisors brooding over a map and Sile is standing in her Tent. I have played chapter 2 for the last 2 days now and have never seen them. They simply weren't there - anywhere.

I am glad that simply meditating makes them reappear. Still, another annoying bug.

I also tried the save-and-reload-while-looking-at-them routine with both of them:
Sile is there for good, she doesn't disappear anymore.
Henselt, however, is not there at the table upon reload, only his advisors - BUT if I turn around, walk a couple of steps and look back at the table again he is suddenly standing there, looking at the map, together with his knights.