If the bottom of the coffee urns is not fully attached, water/other liquid will leak down into it slowly, and the insulation and heat inside will make it a murky black sludge which will leak out slowly once it gets too full. Make sure the screws at the bottom are fully bolted to the urn. If that doesn't work having a shift call the helpline would be good, cause that would mean the urn itself is leaking.

Edit: spelling corrections