People in the Bay Area are playing an exciting game of treasure hunt, as a millionaire has been hiding envelopes stuffed with $100 for strangers to find, ABC News reported.

The anonymous man, claiming to be between 35 and 45 years old, told ABC News that he's a successful real estate investor who just wanted to give back.

"People complain that the price of real estate here is pretty high and I have benefited from that," he said by phone, after ABC News reached him via email. "I wanted to do something fun for the city of San Francisco, something that would get people excited."

Using the Twitter handle @HiddenCash, the mystery man has revealed that he plans to expand his treasures to New York and Los Angeles soon.

Since Thursday night, he has already given away nearly $4,000.

Social media has been buzzing with excited strangers posting pictures of the envelopes they find, sometimes taped to ATMs, parking meters or under benches.

Richard Rodriguez and his partner raced to a street in San Francisco's Mission district Sunday seconds after the Twitter account gave a hint the cash would be hidden on a parking meter there, he told ABC News.

"It's about 10 blocks from us, so we did a power walk down there," he said. "When we got to the parking meter it wasn't there and we thought, 'Wow, how could anyone have gotten here so fast? Then a guy was driving by and threw an envelope out the window. I guess he didn't even have a chance to drop it off!"

A lottery ticket and $66 was found inside the envelope.

"The money was great but it was also so much fun doing something like this. It was a scavenger hunt," he said.

"It's not a lot of money. It's not going to change your life. But the camaraderie it brings out in people is a lot of fun."

The anonymous donor says he'll keep hiding the cash -- usually $100 a pop -- "indefinitely."

"I'm giving right now about a thousand a day," he said. "For me, that's definitely manageable. I know that for most people it's more than they earn, but I'm in the top 1 percent. I can keep doing this."

The mystery man hopes his game will encourage more people to show random acts of kindness, he said.

"One example is you go to the toll booth and pay for the five cars behind you. Well, let's say the person behind you has more money and makes more money than you," he said.

"You're still doing a good thing because maybe they're having a bad day or a bad week and you've lifted their spirits and brought a smile to their face. People that don't need money, per se, can still benefit."