According to data from Glassnode 1.8 million bitcoin have not moved for more than 10 years. As you may know bitcoin is not completely anonymous, unless it has never moved. There are theories about the owners of these coins, as they belong to bitcoin's early history. Some believe they belong to Satoshi himself, and others believe they belong to early contributors like the late Hal Finney.
The stagnant coins amount to more than 20 billion dollars at current prices and they represent about 10% of circulating bitcoin, which will top at 21 million.
What would happen if the coins owner suddenly decided to liquidate his/her position and sell all of them? A big sell would definitely have an impact in price to the downside, but BTC should recover in due time as people take the opportunity to grab some more, we at Crypto Jedi certainly would.