I have been thinking about this and while I do not know and may be wrong I would think the issue here is probably the money that is left on a daily bases during your stay rather than what you leave after you check out. If it is on the desk or dresser, vanity or whatever it could be money that you have simply left out. They might not be able to take it because there is no certainty that it was intended as a tip. However, if you are vacating the room and have checked out and left say $5 or $10 for your stay that could safely be determined to be a tip regardless of where it was left. If it is a larger sum of money than it might be prudant for the hotel to hold the money for a certain period of time to assure that you didn't accidentally leave it behind and later call to retreive it.

In any case, it might be easier to simply put the tip on the bed or wherever under a note that says Thank You... or under a note with any specific requests that you might have for that day. In fact, the more considerate and polite you are about your special needs or requests and if accompanied by a fair gratuity the more service you might get.

Like I said on a previous post, I will be putting out daily tips based on the point that the person who makes up my room on Monday may not be the same person who comes in on Tuesday and/or Wednesday. Also I will put it on the bed or otherwise make sure that it is clearly a tip. It is not a big deal to make sure your intentions are clear.