
Now that I knew the exact location, it was just a matter of time until I encountered Metal Glyphs, too. It took me 54 times for the first one and 83 times for the second one to spawn. At least it's understandable why I was struggling: It can - as I feared - only be encountered in a very specific location, and our experiences indicate a probability somewhere between 1-2% per battle. Although I had long forgotten about where I had found the "sparkle" on the world map, I'm sure I tried my luck in this area as well, but I probably gave up after 50 fights, since I still had to cover practically the entire rest of the game.

Now that I think about it, I should have investigated this area a bit longer, since one of the few random mob griffins (if not the only one) can be encountered here (Mysterious Griffin, no. 159). This means the game did something similar like in the "danger zone" southwest of Anemoss Town, which seems to be the only location where Metal Golems (no. 143) and Golden Golems (no. 80) can be encountered. Monsters of a similar type (palette-swap) seem to stick together. And hindsight is 20/20.
Well, at least the mystery is solved. In the future, I will have to be more thorough - and especially smarter - if I ever face a conundrum like this again while playing a JRPG.