The mysterious philanthropist known as @HiddenCash is urging people participating in his or her Twitter-driven scavenger hunts in the Bay Area and Southern California to be careful.  

In an open letter, @HiddenCash says there have been large crowds lately and there's concern that someone could get hurt. 

@HiddenCash says there is no political, business or religious agenda, he or she just wants to bring a smile to people's faces. and that keeping it going depends on it staying safe and positive.

Check out the letter, here.