I am distrustful of everyone. When I bring my laptop and other valuables I always lock it up when I'm not in the room. Even if I put the do not disturb card I still lock everything up. I was in vegas 3 weeks ago, I also made it a point to check the door when I left and pushed it in to make sure it was secure and closed.
as others have said it is possible that the do not disturb sign was taken, she should ask her daughter if the sign was still on the door when she came back. I would file a complaint if the maid came into my room and cleaned up even though I had the do not disturb sign out.
Part of the problem I see is that the maids keep the door or multiple doors open when they are cleaning, it is easy to slip into another room and make a quick snatch and grab. that's why I always lock everything up when I'm not in the room. even if I go down for a quick bite I still lock everything up.
too bad about your daughter and her loss. put it down to a learned experience I guess but it was an expensive lesson.