In addition to the 12 hide types for Level 1, Level 2, and Level 3, one more, final available type has been hidden and listed, if not yet found:
Letterbox: Pharr Mill Historic Site - OU0AFC
This makes 13 types hidden. The 14th type - challenge - does not seem to be available for me to hide; however, my "unknown" hide for Level 3 was a challenge hide, for what that's worth.