There are some interesting posts in this thread by trader422 about hidden orders being sniffed out by HFT (methods other than pinging with a couple shares):
http://elitetrader.com/vb/showthread.php?t=255059&highlight=hidden
Thanks a lot. Didn't read this before.
trader422: "Later, I learned that the predators know immediately, if they care at all, where the hidden NBBOs are ... in fact they invented a special order type (NYSE calls it an ALO order) which basically has the purpose of allowing predators to sniff out the hidden NBBO quickly and quietly. Of course, they can also do old-fashioned pinging with 1-lots to find the hidden NBBO."
i googled ALO and came up with this:
https://www.sec.gov/rules/sro/nysearca/2013/34-70637.pdf
(proposed order rule changes by "self regulatory organization" NYSE Arca)
"An ALO Order is a limit order that is accepted and placed in the NYSE Arca book only where the order adds liquidity"
So HFT can use this order on all levels to sniff the hidden. If it comes back as rejected they know that an order sits there. If not, they just cancel it themselves after a few millisecs.
there's much more in that pdf. apparently there was an "don't arb me" order modifier.
Have to agree with Lewis now. The markets are rigged. Hopefully the SEC will act now after this gets dragged into the spotlight.