I had something similar happen when I unequipped the wishbone after finding a silver vein's location but without actually uncovering it.

At the time, I suspected that the veins don't actually spawn until we uncover it by breaking through the rock at that spot/depth.

After reequipping the Wishbone, I found that the nearest silver vein was some distance away.

Since then, I always make sure to genuinely uncover, and hit, silver veins before unequipping the Wishbone or leaving the area.