at the moment we use codes that only we'd know on the keys, e.g. "fish" because there is fish on the bathroom wallpaper so that if someone found a key they wouldn't know where it's for,keep a spare key locked away in a safe in our office, as well as trying to only take what we need. anything else we can try and do? even though there is a minimal chance of losing someone's key and the finder knowing where it's for, i can't imagine there is much worse then having to call a customer to say we have lost it so we try and be as secure as possible.