High Frequency Traders pay exchanges for Order Flow. From the...

High Frequency Traders pay exchanges for Order Flow. From the book Flash Boys - 'By purchasing what it expects to be uninformed order flow, a market-maker can buy at the bid and sell at the ask with less risk of trading at a loss than with an informed trader who knows that the market is mispricing the security. Thus, market-makers who pay for order flow can capture the spread while reducing the risk that the spread is too narrow to adequately compensate them for the risk of loss.'

Even though your stops are held by the broker, brokers sell your data to HFT firms, it’s called Payment for Order flow. So it is perfectly conceivable that your broker is letting HF traders 'see' their books.