I don't know. Maybe this is an unpopular opinion. Maybe a wrong opinion. I just can't see leaving leftover food or drink as a tip to staff. It just seems awkward to me.
The following is not the same, but thought I would share. We were once at a table where a guy ordered an extra serving. When the plate was delivered to the table he said it was for the wait person. The waiter expressed thanks, but the look on his face said something entirely different. What the guy was thinking, I don't know.