From the photos this appears to be on the exhaust side of the engine. If so then has to be a part of your EGR system.

First look at your EGR valve. Is there a hose missing? If so this is likely where it is supposed to go.

But if your exhaust manifold has no EGR valve then some previous owner has swapped your exhaust manifold for a Euro version which has no EGR valve. This is a popular thing to do on the M110 if you can find a Euro manifold. They came to the US on gray market cars so they are not everywhere, but a lot of 280sl were imported this way and none of them had EGR valves. So when they hit the scrap yard savvy Mercedes owners know to grab the exhaust manifold.

If this is the case then just plug the hose. If fumes are not being sucked from it then it serves no reason to be there. And the engine would run fine without the EGR valve. Many people think they run better without them and that's why they go to the trouble to swap the manifolds.