I’ve never seen icing without some kind of cloud or mist. When it has happened to me it was a surprise, but when I switched to one of the external cameras, and looked around I could then see I just passed through the edge of a very thin cloud.
That said, when I was flying low level across Tasjikistan the other day, I was flying low to avoid clouds, but in some spots this was impossible as mist/fog was pretty much down to ground level. I even passed through some thin spots of rain as well, and when I checked the thermometer the OAT was maybe -2C. But no ice. I feared the worst, and that I would have to either land, or cheat my way out of it, but I never got any icing on that trip, and it was always below 0 when crossing the more mountainous regions.
I’m not sure how much colder it needs to be before this occurs.