Earlier quoted context omitted.
Genuinely curious: Do you have any more background on this? Who are these rich people that are not listed in Forbes?
This is an ongoing curiosity of mine. One really cool example is Mubarak of Egypt who became wealthy from corruption. When he lost power he had some wealth exposed and he was richer than any known person in the world. https://www.forbes.com/sites/clareoconnor/2011/04/11/egyptia... It's very likely that the richest person in the world right now is Vladimir Putin.
However, after searching for follow-ups on that story, I wasn't able to find any news article saying that any significant (>$1B) amount of wealth was found. There were $664M that were frozen in Switzerland, as well as an undisclosed amount in the EU [1] (but both freezes were annulled in the meantime). Additionally, there were some properties and luxury cars he owned in Western cities [2].
All of these (and some generous extra for wealth he was able to hide) would add up to wealth in the order of billions, but most likely below $70B, and definitely below $700B.
[0] https://dailynewsegypt.com/2018/11/25/the-years-long-tale-of... [1] https://english.ahram.org.eg/NewsContent/1/64/396228/Egypt/P... [2] https://cifar.eu/6-years-later-mubaraks-billions/