What is it you are trying to hide?
When you place a market order, you accept whatever price your broker gets for you. Usually, it is the lowest offer - if you're buying - or the highest bid - if you're selling. However, there's no requirement for them to do that. For an over-the-counter stock, your broker's trade desk may not prefer to do business with the "best price" market maker. That's the big stink about back office fees. I've placed orders matching the bid/asked price and watched others filled ahead of me many times. A call to the broker gets a denial, but usually results in an execution shortly after I hang up. A coincidence, I'm sure.
The only "hidden" orders I've heard folks talk about are "stop loss orders". If you place one with your broker, the market makers can see it and may dip or bump the price to fill it for you and then return the price to the previous level. To avoid this, the recommendation is to write your stop loss figure on a post-it stuck to your screen, but don't tell anyone. I use a service called Tradestops that tracks my portfolio and sends me an email when a stock has hit the stop loss I've set (or accepted the one it's suggested).
Oops. I forgot about the block trading desks. That's the kind of trade you usually initiate over the phone. Say you want to sell a big block of stock, but you don't want to destroy the market. Your broker's desk traders look to cross match trades or, accumulate smaller orders. I don't really know how. I've only done it a couple of times. It is nice, though, to be able to sell or buy large amounts (read, tens of thousands or even millions of shares) without significantly affecting the market. It can't always be done, but they try hard.
Hope this, or 8's reply, answer your question.
Dino
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