@Hansmex - I tried but I think I was really in geometry in school I get messed up in the 3D world. Anyway, I haven't taken time to test all those new possibilities. I think, I will get a good result.

@Keet As understood, there is no way in SH3D to make a whole into an object. I have to do this in another program

What you explained, will be a great help in some situations. But this time I meant, how can I find objects, which were made invisible. But now I am thinking, they still must be in the list in the bottom left corner. (and indeed they are)

So thank you for the great support.