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.