380,670,753 Feb 01 09:37 AM Huobi
1DUb2YYbQA1jjaNYzVXLZ7ZioEhLXtbUru 329,339,572 Feb 01 09:37 AM Bittrex
1Co1dhYDeF76DQyEyj4B5JdXF9J7TtfWWE 270,000,000 Dec 21 '17, 06:14 AM Poloniex
...
These exchanges presumably had to buy the tether with real $$ in order to list tether on their exchanges in the first place. Whether the money tether received is still tucked away or not is anyone's guess but unless tether gave all these exchanges free tether, your $50 - $100 million doesn't make sense.
1DUb2YYbQA1jjaNYzVXLZ7ZioEhLXtbUru 329,339,572 Feb 01 09:37 AM Bittrex
1Co1dhYDeF76DQyEyj4B5JdXF9J7TtfWWE 270,000,000 Dec 21 '17, 06:14 AM Poloniex
...
These exchanges presumably had to buy the tether with real $$ in order to list tether on their exchanges in the first place. Whether the money tether received is still tucked away or not is anyone's guess but unless tether gave all these exchanges free tether, your $50 - $100 million doesn't make sense.
In an exchange, you're generally trading with another customer, not the exchange. (The stock market has market makers who are obligated to step in when there is no opposing party. I don't know if crypto exchanges have the same thing or not. If so, then some small portion of transactions will be with the exchange.) Further anything you hold at the exchange is in the exchange's wallet. The above addresses are likely the exchanges' cold storage wallets.
So - this implies that most of the USDT on exchanges was brought there by customers. A tiny portion may have been bought by exchanges as a starter pool. Some unknown portion may be profit earned from the spread.