It very much looks like something I'm experiencing these days with older props: they're 3Delight, and sometimes upon conversion become emissive. So in that case, I'd look through the shaders of the surfaces that turn out brightest in your render, check whether it has an entry "Emission Color" on the surfaces tab.
If it doesn't, it's a 3Delight shader, so you'll need to convert it.
In your smart content, pick the files tab, go to shaders -> Iray, and apply the !Iray Uber Base.
The surface should now have an "Emission Color" entry. Make sure its color is BLACK (no emission).