I like it when my players are rewarded for looking at things, investigating their environment. But I don't always have a secret room where they are looking.
Inserting secret rooms where there were none before the players look is very Quantum Ogre. In general the idea should be that there is a "real world" the players are investigating, and if they roll a 20 to search or search really well through description in a room that doesn't have any secrets too bad! They learn there are no secrets. However, I think these rooms are meta enough to split from this issue. I'm also playing in RAPL so having weird backstage rooms or spaces in between the dungeon feels correct.
So here are some meta rooms I might have the players find
Roll call room: This backstage room has a series of scribes noting which players and characters have been on which adventure or session. They have Yule Logs, Dinner Rolls, Scrolls, perpetual "rolling" machines, dice, Masks (for Roles), and other jokes. The players can talk with the scribes who are slightly concerned to see them there and are having trouble noting who is who. The scribes know the players and call them by their real names or get them and their characters confused. The scribes are getting a few recursion issues with the players in the room but assure them its nothing to worry about. They give a treasure to whichever player has the best attendance.
Backstage: The NPCs or monsters from an adjacent room are chilling here, nibbling on a rolled ham and cheese plate, drinking mediocre coffee, and making small talk. Even their worst enemy is not hostile here, the masks come off and everyone knows there are just playacting. Feel free to have the NPCs or monsters gripe to the players about things that are annoying you as a GM. NPCs might let something slip, or you might be able to find out monster weaknesses from how they act. There is a "script" of what will happen next buried on a table under some bills and advertisements. This includes a few rooms of descriptions and GM notes from the dungeon. Reading it makes the players' heads hurt.
Treasure Room: Labelled as such from the outside, in an ancient and very impressive looking script. I imagine the character wiping the accumulated dust of ages to find this text. Inside, beautiful paintings, prints and drawings of the players on their adventures, focusing on when they built their friendships. Bonding treasure if the players show true friendship.
Discord: A completely silent man, who seems to be trying to talk/speak but no sound coming out.
Something about switching discord servers or Google Hangout or something to hear him. He has many helpful hints.
