It turns out that in the US anyway, sulfites on salad bar vegetables have been banned for oh, just the last 24 years. I don't know about the EU, but I'd be a little surprised if your regulations were less restrictive.

Dressings could still contain sulfites - although I rarely buy pre-packaged salad dressing, I can't remember seeing sulfites on any label - but from what I'm reading, it's possible.