Last time I stayed at VWL (which was 2008, so maybe things have changed), I saw several housekeepers over the course of our visit with garbage bags full of partially used bottles of detergent. I had never thought about it, but I think a lot of detergent gets left behind ... nobody does enough laundry on vacation to use an entire bottle (do they?) and most people aren't going to bother carting an open bottle of detergent home in their luggage.