I've left behind 15 or 16 over the years...that didn't come back. College town. I have stickers with name address and phone number on them, but still they disappear from time to time.
As Mike said, most people are pretty honest and either bring them back or call me. I remember one lady told me the NEXT YEAR when we returned to clean again that she "saved" two fans we'd left the year before. Still, I was grateful for the favor.
A few years ago during a brutal heat wave I loaned out a few for people without air conditioning. Darned if I didn't lose a couple for that.