Diesel Fuel Filter At the Rear: Explanation.
Mondeo fuel filters come in two flavours, either
near the engine (maybe post 2013) 2007 to ... or at the rear
adjacent to the fuel tank.
Where the filter is at the rear near the fuel tank
opening the drain plug will not always result in draining the filter.
The filter might be lower than the fuel level in the tank
and fuel will just syphon from the tank.
See 2:35
https://www.youtube.com/watch?v=Po4BddwSJ1Q
Worse,
Simply disconnecting the line from the filter to the pump
means that fuel can just run out of the supply line.
Which means that air will take its place.
This wouldn't be a problem,
except the fuel pump is not self priming.
The pump is lubricated by diesel fuel,
so attempting to start the engine
could mean the pump is running dry.
Best to avoid the possibility in the first place
by promptly plugging the fuel lines.