Secret maps
Thalion office
There is a secret map which reflects the Thalion office. It consists of several rooms which has some chests with most of the game's items. You can also find a few secret party members which embody some of the Thalion members.
To enter the map you first need the item "Netsrak's staff". You can find it in a chest in the secret room in Grandfather's house (see below).
With the staff head to the town of Spannenberg west of Grandfather's house. There go to the north east where you will find a small alley. At the west end of the alley there is a sign on the wall which says "Karsten was here". It is a reference to Karsten Köper, the game designer. Also "Netsrak" is just "Karsten" reversed. Using the staff when standing in front of the sign will teleport you to the secret map.
In Ambermoon Advanced you will also find a guy called Alex in the hallway of the map. It is a reference to Alex Holland, who was not part of Thalion but is one of the biggest preservers of the game and rescued the only english version of the game. He runs the Thalion webshrine. In contrast to the other characters on the map, he is only an NPC with some text. You can't invite him into the party.
Secret room
In grandfather's house you can enter a secret room behind the left fireplace in the big dining room. To do so you just have to use the bucket next to the fireplace to put out the fire and then enter the chimney.
There are several chests. One of them is locked. You can open it with a lockpick or the pick lock skill. In it you find some lanterns and Netsrak's Staff which is needed to enter the secret Thalion office (see above).
In Ambermoon Advanced you will find a bookshelf on the upper left of the map. When interacting with it you can get a brief summary of the changes in the expansion.