I cannot speak for other specialties, because everyone knows that there is undeclared work happening everywhere. However, in our field, IT, it is impossible, especially because we work with large companies, because we work on large computer systems.
Earlier on, I was referring to equipment. The only thing we make use of is our brain power to be able to use... We use the equipment that is there, but it is thanks to our brain power that we operate things.
So, it is impossible for clandestine work to be happening in our field because we are always billing the client. Everything is set up in this manner. The same applies to our expenses, everything is there. For our part, undeclared work, as far as I'm concerned is non-existent, for sure. I would say that for most IT professionals, very little of that must be happening, if at all.