Standard place for the maid tip is on the pillow of the bed. Leaving it on a table or something without it being conspicuously marked makes them afraid to take it sometimes because they don't want to be accused of stealing money that may have inadvertently been left out.
I'm super surprised at the younger folks not knowing about tipping the maid. Not calling anyone out on it, but I'm just intrigued as to when or how that started. As a little kid, I always saw my parents leave a tip for the maid, and they immigrated from a country where you don't tip, so somehow they learned about it as a common practice, but somewhere down the line that has apparently reversed, and I'm really curious as to how and why.