I did not understand the philosophy of the hibutsu (secret Buddha) until I chanced upon one a few years ago.
To treat a Buddha statue as a hibutsu is an essentially Japanese phenomenon. The statue can be sometimes made open to the public (Gokaicho, or "unveiling of the statue"), which is, and has always been, a great time of festivities. The statues which are occasionally made open to the public (even if it is once in hundred years) are secret in the relative sense. Other statues are secret in the absolute sense, e.g. the famous secrete Buddha in Zenkoji temple, which became the site of the opening ceremonies for the 1998 Nagano Olympics.
The Buddha statue in Zenkoji is absolutely secret. Nobody has seen it, not even the powerful warlords like Oda Nobunaga (1534-1582) .
Even when one can worship a secret Buddha statue, one is not allowed to take a photograph or make a drawing most of the time. If you are lucky enough to glance upon one, the only thing you can do is to try to keep its image in your memory.
The secret Buddha experience makes one reflect on the onceness of life. Some things just happen and then pass by for ever. You cannot capture the essence and keep it for record. Life happens to you once and for all, never to return.