A bit more detail would be helpful. Where exactly have you encountered these hidden rooms?
In general, all rooms have doors which can be opened either by clicking on them, using a key, wearing an amulet or other quest item, activating a floor tile or throwing switches.
The only other instance I can think of is some quests where areas or rooms are only accessible via teleporter pads.
Hope this helps
[ 01-22-2002: Message edited by: Mouse ]