I see by the term "chop" we are dealing with an Asian culture ("chop" being equivalent to a signature stamp for an individual or a department.)
The primary thing is to remain calm.
- Ask the individual politely how he came by the copy - did he make the copy or did someone give it to him? If so, who?
- How has he individual been using the copy? - does he do work on a product to which the copy refers?
- What, exactly, is the nature of the document? Is it top secret? Does it refer to a process or is it something like a detailed drawing used for inspecting the part? Perhaps it's something like a price list?
Once you know who and why the copy was made and how it was used, you can determine whether your system is lax or whether a trusted individual intentionally found a way around the system. THEN you and the bosses can decide whether to change the system or the trust level of the individual!
Note:
If the system to get a copy for an employee to do his regular work is too stringent or complicated, the worker will try to find a way to make his work more efficient. This mentality goes for documents, tools, work gloves, etc. The worker doesn't intend to humiliate anyone or steal; he just wants to make his own work life easier. If this is the case, there should be a frank dialog with workers to make work more efficient without introducing a possibility of using an obsolete document and thus creating nonconforming work.
If these documents contained trade secrets which could have been leaked to outsiders, you have a different problem.