@Laura2592 the guests have access to the W/D??
If so, rather than trying to patrol their towel usage, maybe you could implore them to at least start a load of laundry before they leave, since presumably, they've used these over time and not all at once on the morning of departure??
that would give cleaners some headstart.
Or ask them to complete a whole wash/dry cycle...
"please begin the laundry process so that the cleaners just have one remaining set of bed linens/towels per guest to complete. Much thanks!!"
they likely won't all comply but if even some of them do, would that be a win for you?
I do personally tend to worry about guests doing laundry though. I've seen guests do some pretty knuckleheaded stuff so I try not to trust any crucial tasks to them.
~ this is why I'm glad the Barn is in the backyard.