On my 220d, the third line runs to a little rubber port that hides behind the bumper at the passenger side rear corner of the car. Perhaps yours is similar. It is visible of you look for it behind the bumper, but you might not notice it otherwise. I think that the black plastic box is designed to capture any liquids that might get forced up the vent lines (when parked on an incline or on a road with a lot of crown). I had mine still hooked up, but not at a high enough location in the trunk. And liquid diesel would get leaked out of the little port that I mentioned above. With the black box higher in the trunk, no liquid diesel ever leaks out of the port. So I would discourage you from simply venting to ground, because you will probably have a gas leak as a result. Leaking diesel is not great for the environment, but leaking gasoline could be a real safety concern.

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1968 220D, w115, /8, OM615, Automatic transmission.
My 1987 300TD wagon was sold and my 2003 W210 E320 wagon was totaled (sheds tear).