Your reasoning suggests the terms of the copyright license do apply when used as an internal tool (you'd be free to keep the code hidden otherwise) in which case preventing anyone from publishing any work derived from code under a copyleft license seems impossible. At the very least the GPL doesn't seem to allow you to create a derived work and require people to keep it secret even if it's just internal to the office (though I guess they could keep it secret willingly, I just see no way to legally compel them).
This seems different from something that's purely personal use. I don't think it makes much sense to force a program to show a notice its GPL licensed if you're the only one using it for instance. In fact that seems to run counter to the intent of free software.
It's possible that legal systems treat this situation differently, but I think it makes far more sense if you don't need an additional license to use stuff that's already published by the copyright holder until what you're doing starts to go beyond just personal use.
This seems different from something that's purely personal use. I don't think it makes much sense to force a program to show a notice its GPL licensed if you're the only one using it for instance. In fact that seems to run counter to the intent of free software.
It's possible that legal systems treat this situation differently, but I think it makes far more sense if you don't need an additional license to use stuff that's already published by the copyright holder until what you're doing starts to go beyond just personal use.