Wait, I may have that wrong. I remember an arch, but there are several different designs you can see in photos online. The more specific descriptions seem to describe a rectangular section set into a larger gate.
I believe the (relatively more famous) arch one is above the gates of Auschwitz I ; while at Dachau and Sachsenhausen it was indeed written into the postern gate (the small one inset within the larger one). Cite.
What I was semi-humorously trying to imply is the potential for, a few decades down the road, for neo-nazis to ask people to prove that that message was placed there, because there’s no sign for proof.