I had this once happening to me. Almost worse, as the guest just entered the room when I exited the bathroom.. well, you get the point..
VERY expensive week stay (during a exposition) - paid by company, so I wasn't really interested in getting any money for the room back as it would have gone for the company anyway - not that they offered that, but all they eventually - after I went down to the lobby - offered was a free dinner, which I eventually took. Their restaurant was mediocre, and I later found the 2 beers I had with the meal on my room bill.. yes, THATS a sure way to get a couple of very loud words from me at checkout.
As my options to take "revenge" on the hotel were pretty limited, all I could do was giving them a one-star review on TripAdvisor.. not that I would ever have booked that property on my own dime anyway, it was 5* price for a 3* product (which called itself 5*, of course) otherwise..
I think getting back one night, or a fully paid dinner, including a very seriously meant "We're sorry" should do it. Anything less shows that they don't even care about such trully "we ....ed it up" event..