Oil Slick
A oil slick object or icon found on Murdoku case maps.
Explanation
Oil Slick appears as a board icon on the crime scene grid. When a clue card mentions "oil slick", locate that icon on the map and apply spatial rules such as beside, south of, or region constraints.
Example
If a clue says "beside the oil slick", find the oil slick icon on the grid, then check the four orthogonally adjacent cells in the same region.