According to Brian Cates, Elon Musk “didn’t buy 9.2% of the shares at once. Over time, very slowly, probably over the course of more than a year, he quietly and surreptitiously bought small amounts of shares from Twitter stockholders. 

Nobody tipped off Twitter, they didn’t see him coming. He took them completely by surprise.  

If he’d bought 1% or 2% of the stock at once, that would’ve been very hard to hide from prying eyes. 

So he bought far less than 1% at a time.  Maybe around .033% at a time.”