From Dark and Darker Wiki
< Where does skull key drop in inferno from “loose loot”?
Loose Loot = On the ground, on tables, bookshelves, in and around statues, and otherwise hidden throughout the map. In High-Roller Inferno it has a 1% chance of being selected from the loot table of loose items such can provide, in normal dungeons inferno it has a 0.5% chance of being selected. It is possible that it can only be found in certain loose items spots in the inferno map but im not sure. Please note that the wiki is Work In Progress and future entrys may have the awnser you need, i recommend checking the page for the skull key (https://darkanddarker.wiki.spellsandguns.com/Skull_Key) and the page for the lootables (https://darkanddarker.wiki.spellsandguns.com/Lootables). I hope this helps any adventurer with the same question as you. See yall in the dungeons.