UPDATE 15th August 2017: Bungie has said Destiny 1 silver on Steam was an "error" and Valve will issue refunds.
In a tweet, Bungie community chief David "Deej" Dague apologised for the confusion.
Of course, that tweet doesn't explain why the Destiny 1 transactions were made available on Steam in the first place, with Destiny 2 set to release on Blizzard's Battle.net rather than Valve's platform.
ORIGINAL STORY 14th August 2017: Ever since Destiny 1 was announced in February 2013, fans have wondered whether Bungie's online shooter would come to PC. While that remains unlikely, you can, bizarrely, buy its in-game currency on PC.
Eagle-eyed users on NeoGAF spotted Destiny 1-related listings on SteamDB - nothing new there. But what is new is you can actually drop real world money, via Steam, on Destiny silver. We've tried this ourselves and it is indeed possible (although as you'd expect, you don't actually get anything when you buy it, save a new item inventory notification).
In Destiny 1, you buy silver with real world money, then use the silver to buy goods for your character at the Eververse Store. Why would you be able to buy silver for a game that doesn't exist on Steam? We're not sure, but we've asked Activision for comment.
Meanwhile, PC gamers will be able to play Destiny 2 on PC, not via Steam but via Blizzard's game launcher.