The Twitter treasure hunt began on Friday, when the user started sending out clues.
Twitter went into a frenzy, with the user gaining more than 85,000 followers as of Tuesday afternoon.
This morning the account had over 115,000 followers.
Reportedly, the HiddenCash creator has told the media that the giveaways are a "social experiment for good".
Envelopes chock-a-block with cash have been taped to various locations around the city, from lamp-posts to parking meters.
The HiddenCash creator hopes that winners will use the money – sometimes $100 or $200 – for good deeds.
One post from the mystery person said: "Frankly, a little overwhelmed with requests for media and cash. It hasn't even been a week. Wow."
The generous Twitter user has said: "@HiddenCash has no political, religious, or business agenda. @HiddenCash is about love, kindness, and generosity."
The Twitter feed has promised that more cash giveaways will soon take place in other California cities, including Los Angeles and San Jose.