If they haven't paid for the code, and it includes no GPL'd modules, and it is not mentioned in any contract then I don't think you have to disclose.

I don't think there is any requirement for disclosure, even if the code does use GPL'ed modules. The GPL requires that if you distribute software, and that software is a derivative of GPL'ed software or the software links against a GPL'ed library, then the source for the software must be made available.

The GPL does not require you to distribute software you otherwise would not