There are alot of multi millionaires and billionaires that we don't know or aren't on forbes list. There could be one guy out there richer than bill gates but is quiet about it.
It's very hard to be richer than bill Gates w/o being noticed. Either you ceo of a major company....or have big money invested in companies like buffet where you and/or holding company has to be transparent above a certain percentage of ownership/shares.
Richer than bill Gates and no one knows....he/she in the black market and alphabet boys is checking accounts. Hell, If you buy a house six figures plus in cash in the USA you get investigated by the IRS and have a waiting period.
It's hard to keep billions quiet unless you were established before said point.