The best way to handle these encounters is to have the employees trained for this kind of encounter.
The information of the training should include:
- which programs to be shown on the computer screen, and which not;
- close "secret" programs whenever non-employees are around;
- which questions they can answer directly, which questions should be forwarded to a superior;
- how to answer questions without giving away sensitive information;
- define what is "sensitive information".
This kind of training should not take more than 1 hour, since the information is pretty much obvious. The preparation of the materials for the training might take more, of course.
Once that is implemented, anyone can ask anything and look anywhere, there will be nothing available to compromise the business.
The above is necessary when dealing with strategic customers, which must be kept happy at "all" costs.
For regular customers, just tell them that it is the company policy to keep customers away from the staff.