I just discovered how that errant group may have formed within my hook tip.
Curviloft; when it fails to form a surface, even though it shows a black pre-form surface, still generates an empty, or perhaps an almost empty group that SU interprets to be as a non solid. SI will correctly say it finds no errors, but SU will not see it as a solid.
The only way out, is if you know you do have a valid solid after a Curviloft fail, but SU says not, then edit the solid and do a sweeping left select to select everything. Using TT's selection tools, select only groups, and there it is. Just hit the <delete> key to get rid of it. You cannot select just that empty group. If you have other valid groups in that selection, then deselect them first.
I'm posting an edited version of this in the Curviloft thread.