This happened to me yesterday. It was unreasonable for me to play on, so I was going to resign but my opponent was not at the board. They probably left the board to watch another game, waiting for my next move.
Question: What is the correct way to resign if my opponent is not at the board?
I stopped the clock, wrote 1-0 on my scoresheet (I was black), and started resetting the pieces. But this doesn't strike me as an officially correct thing to do.