You may have been thinking “porous”. Which is a sensible term when refering to a border. I have never seen anyone use “opaque” (because it doesn’t really apply) and suspect it’s down to an inept translation from some statement made in German.
‘porous’- - yes, that’s a better word. I just didn’t think of it at that hour -like now- it was late and I hurried my post. Gotta pardon me and some of my posts now; but can’t sleep very good some nights. I use o2 and it’s getting a bit harder to breath lately, so I am up at all hours. Thank you for taking the time to post because that word porous is exactly the word that slipped my mind to describe our borders.