When the engine is shut off the cooling system pressure exceeds that of the fuel system and water can seep into your fuel filters and fuel tank. The water can cause rust and corrosion in the entire fuel system. For this customer we also replaced the fuel filters, drained and cleaned the fuel tank.
In the top left picture you can see part of a rubber seal that was being sucked into the injector.
It's actually coolant dripping through the crack and into the injector cup.
It's easiest to see in full screen mode.