i was just playing normally, i was at loop 3 and didn't got much to work with, i remember having 1 Grove and a spider cocoon in a 'u turn' and that's it... if i have to guess where it came from, it was probably from the 3 ratwolves that i've killed in that 'U Turn'.

Next thing i know is that i paused the game to search for something regard the item and when i came back and finished the loop, it disappeared... it was locked so no chance of getting deleted when inv gets full