We see Bill Cipher in the slide that Webby shows Lena about the other money bin in the episode “The Other Bin of Scrooge McDuck!”.

Other Bill Cipher sightings in DuckTales:

Bill Cipher #1 from “The Living Mummies of Toth-Ra!”

Bill Cipher #2 from “Beware the B.U.D.D.Y. System!”

The Gravity Falls series May have ended, but Bill Cipher is always watching us.