There are many locations in BFDI that have been mentioned by characters but never actually witnessed on-screen.
In "Rescission", Announcer's Diary mentions that he isn't from Earth, implying he's from another world.
At the end of "Return of the Hang Glider", the speaker boxes come to collect Announcer. It was actually mentioned for the first time when Teardrop Speaker Box says that BFDI wasn't a success and that Announcer must return with them and it was "time to go home."
The library is a place that supposedly exists in the Plains. It is mentioned by Book in "Get in the Van" when Pencil drives through the place, there is apparently a "rule" in which Book is allowed to slap Match every time they pass through this library. In return, Match can slap Book if they pass through a matchstick factory, which does end up happening in "The Long-lost Yoyle City".
In "No More Snow!", as Ruby was pressing buttons on Evil Leafy to escape from her, she pressed buttons that screwed up the process, killing Pencil, and exploding Match and Ice Cube. After Ruby presses the yellow button, Pencil scolds Ruby and asks her if she went to Gem School. Ruby says that she did not go there, but Book mentions that Gem School is only for diamonds.
- There is indeed a recommended character called Diamond, and one of Ruby's sisters are named Diamond, but it is unknown if they attend/attended that school.
- If Book is correct about the information that only diamonds attend, it is possible that there could be diamond clones across Goiky.
- It's never been seen on-screen, which leaves its existence a mystery or unknown.
- Only members of FreeSmart have seen/mentioned Gem School so far.
- This is the first place that has to do with school, the second being the EXIT.
The Burn Center seems to be a place where one would go to recover from being burned or roasted (being subjected to ridicule).
It was first mentioned by Four in "Why Would You Do This on a Swingset", when asked where X was by Cloudy. X presumably went there because Four roasted him by telling him that at least he's "not the one that doesn't know their value".
Limb Reattachment Center
The Limb Reattachment Center seems to be a place where one would go to reattach limbs they lost.
The Massive Department Store
The Massive Department Store is an unexplored location that sells various different products.
It is first mentioned by Foldy to which Yellow Face promotes items. It is known to sell products such as Cheap Plastic Green Toys, Fork Attractant, Non-Slip Shoes So Ha, Revolutionary Earmuffs, Bubble Transformers, Box of Paper Slips, Revolutionary Headphones, gumballs, and various other products. Yellow Face's Warehouse could be The Massive Department store because it is also owned by Yellow Face.
The Tropical Rainforest is a location briefly mentioned in the epilogue of "Fashion For Your Face!" located somewhere in Goiky. It is a tropical rainforest that apparently spans "millions of acres", which was deforested by Flower in order to obtain Goiky bamboo, which is considered endangered. The bamboo was then used as fibers for Flower's line of sweaters. The deforestation of the Tropical Rainforest led to "ecological devastation worldwide", according to the epilogue of BFB 23.