Very common to have such shakeouts, especially in the first 1-1.5 hours of the regular trading session. 90% of the time clusters of stops will be below/above round figures and/or previous bottoms/peaks. In your case, it was actually both (3,200 as a round) as well as below the overnight session bottom (3,210). You should get yourself away from those zones when placing your stops or simply trade with them when the market seems weak in breaking those zones. They can't see your stop order, but because everyone is predictable and uses the same stop placement tactics (Which are all around in TA books), it can always be guessed with fairly high probability. If you wonder why the market moves to those zones, it's because it's looking for liquidity. Your order might be just a 2 contracts, but when you group that with thousands of other orders/traders (Even larger traders trading 100s of contracts), you find lots of liquidity there.
From my colleague Joe Easton: When you place your orders on the exchange they become public information, so to speak. You can see these order increase the order size on that price level when placed. This does not mean bots or traders are using that information against you. There are programs designed to read order flow. Traders/programs see support and resistance and often markets will trade just above or below the levels to test the price action.
Surely only limit orders show on the order book as resting orders, yes? Stops become market orders when triggered, i.e., price moving to the stop level. And at that point they're crossed with limit orders in the order book. I usually bracket my orders such that I have a stop above/below market and a limit to exit above/below market. My limit order will show on the orderbook. My stop order will not. Correct, yes?
they might be able to sniff out stop clusters on a statistical basis as most people using TA to place stops do it at the same obvious areas.
The official note is that you can only see limits and not stops. I need to confirm that. My conspiracy theory side says that "they" ( not sue who they are exactly LOL...) know where the stops out. Will make some calls to order routing vendors and find out.