A visitor NDA is a short confidentiality agreement presented at sign-in, used when guests may see or hear commercially sensitive material: product prototypes, client names on screens, deal discussions in open areas. Law firms, agencies and R&D sites use them routinely.
The practical questions are when to require one (by visitor type or by zone, not for every courier), how to present it (a single acceptance step at digital sign-in, stored against the visit record), and where signed copies live when legal asks three years later. Paper sign-in books fail that last test almost every time.