Foolishly I failed to investigate in detail the short piece of road marked ""Where is the border?" on these maps, although I did drive along it, blithely believing that I had driven into and back out of Belgium at the time. My recollection is that there was a change that made this evident but I cannot be certain and, as you can see, the two maps are inconsistent in how they mark the border. This is the Belgian map:
I turned off the L106 and went along the residential street in Aderich, crossing back to the west side of the tracks just south of the "German Farm" (Rückschlag) enclave at Entenpfuhl (= "Duck Pond"). This photo is looking north along the tracks, my car is parked on the side of the road having crossed back into Belgium.
This photo is looking from the Belgian side back down the road into Aderich. It is a residential street all the way up to the rail lines.
A very short distance further north I encountered the south-eastern corner of the enclave and its border marker which I photographed from 3 angles.
The Belgian side:
The top - clearly showing the 90° turn in the border:
The German side
Looking along the road to the north - farmhouse in the background:
...and a video of the area.
Moving up the road now to the north-east corner of the enclave we find border stone 759
The "T" marking on the top is rather strange - it should really be a "V" mark like at the south east corner I would have thought.
I took a couple of short videos around here. It really seems that this doesn't have a lot to do with the Vennbahn directly since the road is in Belgium as well and so is irrelevant as far as the railway is concerned, being separated from the rail lines by a fairly significant chunk of wood/scrub land