Here is the full list of all 6 The Janitor achievements.
Find the easteregg dedicated to Archmeton, a story artist of the game.
Find the second exit in the catacombs.
Find the the first exit of the catacombs.
Find the easteregg dedicated to Marcus Lantermann, a story artist of the game.
Find the easteregg dedicated to Ramona Lantermann, a story artist of the game.
Collect all parts and tools in the asylum.