I think that since 1972, manufactured boats built or sold in the USA have had to show an HIN, usually on the starboard side of the transom, near the sheer. I've never heard of a "hidden" HIN. Is this a new developement?
Interestingly, in my state, Oregon, a HIN is not issued to a one-off boat until the boat is completed. The HIN is issued at the same time as the title and registration. This makes it kind of difficult to hide a HIN.
I believe that an USA documentation number may be issued as soon as the keel is laid (or other phase, corresponding to material type), so that the number may be installed at the builder's convenience.