schrieb:

Having watched gliders from the ground and air, I've noticed that from
directly in front or behind, they nearly disappear because of the very
narrow wings and fuselage.

Sometimes they even fully disappear. Every glider pilot is well aware of
this. Every year there have been a couple of midairs between gliders in
Europe, and that's why most gliders in the Alps are now FLARM equipped.

That said, as I hear (third hand "knowledge"), the glider involved in
this accident was thermalling and thus should have been very well
viewable. And Minden is very well known as glider area with the
appropriate warnings on all applicable maps, so the jet crew was warned
and should have looked out extra carefully.

Stefan