score:4
I'll refer to DJClayworth's answer in the other thread (to whom credit):
The best way to leave a tip is in cash and prominently with a note.
It's best to tip once at the end of the stay. It makes it more convenient for you, and for them. Don't worry about not getting the right person - it all averages out, and tips are frequently pooled anyway. However the rest of this answer applies if you are leaving tips daily.
Addressing your additional question about mistaking left cash: during your stay, if you leave any amount of cash during the day, it's highly unlikely housekeeping will interpret the money as a tip unless it's accompanied by a note. At the end of your stay, however, a note is not necessary: if you leave a moderate of cash (on the order of a few dollars per night) in a prominent place, housekeeping will assume it is a tip.