I always leave a daily tip on the desk with a Thank You note addressed to Housekeeping. I may have change on a desk just inches away from the note with the tip and it has never been touched, but the tip and the note are always gone.

I do leave tips daily as the maids do change, so leaving one big one at the end of the stay, may end up in the pocket of the maid that was just lucky to be assigned to the room on the last day, and not the one(s) that actually worked the room during the stay.