Well, I'll be jiggered! I learned something new again last night.
At the back of the ground floor of the Candlekeep Library, there is a kitchen.
Next to the kitchen, there is a pantry.
On the rear wall of the pantry, there are some shelves.
Below the shelves, there is a secret passageway, with stairs leading down.I guess playtesting does have its merits after all.
It is closed when you first enter the library. It doesn't seem to be detectable by a Thief looking for Traps (most likely because it's been almost completely disabled). But I have found that it can be revealed/opened in two ways: Either by killing the Iron Thronies, or by looting the library for everything worth carrying away. Too bad that you can click on it and it won't take you anywhere.
In a minute, I'm going to send the screenshots to icelus.
Largely-Unsubstantiated Hypothesis: Originally, the Watchers would only arrest you after you
exited the library. This would give you the opportunity to go up to the 6th floor and talk to Tethtoril who, if you were on his good side, offer you a way out of imprisonment and possible execution: Go to the chest in his room and take his Ring of Protection [exactly like every other RoP, just with a different item code], which doubles as a ward key which will enable the party to bypass the protective magics around the secret door hidden in the pantry wall. The party goes through, and the find themselves in the little room that Tethtoril teleports you to in the current version.
If somebody with some more potent modding tools could open up this section of the game and see exactly
what triggers the secret passageway to open, that would be mighty nifty.