Sometimes when you search an area (in a standard location for secret doors, i.e. surrounded by 7 wall blocks) you get the message "You have found a secret door!" but in fact you seem to have found a secret floor.
One of the walls is replaced by a plain floor and there is no door at all, neither open nor closed.
This seems to be a fairly recent issue, I cannot remember having seen it in older versions.
It is probably a bug.