Professor Layton and the Phantom Thieves (partially found Professor Layton online mobile board game; 2012)
Reiton kyōju to seiki no nana-ri no tōzoku (Japanese: レイトン教授と世紀の七人の盗賊 Translated: Professor Layton and the Seven Thieves of the Century ) or more known as Professor Layton and the Phantom Thieves is a Japanese online spin-off game to the Professor Layton series of games and was developed by Level-5 and DeNA Co, and was released on Android and iOS in Japan only in 2012. The game was released as a Freemium, meaning the game itself is free, but people have to pay money for additional premium content. People who registered the game early would receive a unique Luke doll, one of the rare avatar items in the game. The game was announced at Level-5's 2012 Tokyo game show along with other Professor Layton games and was set to release worldwide but never happened. 
Very little is known about this game, except that it wasn't your typical Professor Layton title with its unique puzzles. In the game you create an avatar character, and gather information in order to find and catch criminals and would play out as a board game with different locations from the Layton series.  You can also play with other people online and customize your avatar with different heads, hair, face, and body (clothes). The service was unfortunately discontinued on October 28th, 2014. And everything that was on there was gone with it.
Phantom Thieves cannot be found anywhere online, no copies of the game have been made public since then, only promotional images and very little video recordings have surfaced. Professor Layton Youtuber, "DearestHershel" managed to get a copy of the game and talked about it on his Tumblr page.  The owner of the "Search for Professor Layton Lost media" Discord server would ask DearestHershel if he could release it, but he followed up saying he doesn't want to get involved into legal troubles and said he probably couldn't. Someone from the Discord server named "eneoil" managed to get the files for the game and managed to bypass the login screen, but gave an error due to the service being shut down and is now currently trying to figure out how to access it. Some think that reverse engineering it would probably get the game to function.
On May 27th, 2022. A Search for Professor Layton Lost media Discord server user took a closer look at the games files and said that some of the files are missing and that the game might've relied on those files from the internet saying that without them, the game will probably never function.
- Professor Layton and the Mansion of the Deathly Mirror (lost Professor Layton mobile game; 2008-2014)
- Professor Layton Mobile R (lost FOMA 903i/703i updated port of puzzle adventure series games; 2012-2014)
- Layton 7 (lost production material of cancelled "Professor Layton" mobile spin-off game; 2013-2018)