Some categories of "predatory algos" closely monitor the markets in order to sniff out exactly these types of hidden large orders, so that the algo can trade against them. For instance, if a predatory algo detects that someone is trying to hide a large sell order for INTC by trickling it out into the market in small blocks, it might work to bid down the price of INTC just a bit so that it can pick up those blocks at a discount and then sell them for a profit when the share price floats back up to the market's earlier, non-manipulated valuation...The Matrix, but with money: the world of high-speed trading (via Futurismic)
The final animal in the HFT menagerie that I'll point out on this brief tour is the automated market maker (AMM), which is a subtype of what is often called "dark pools," or "dark liquidity." AMMs like Citadel always stand ready to buy and sell large quantities of assets, and they don't publish price quotes to other market participants via exchanges.
To find out what assets a dark pool will either sell or buy and at what price, you first have to ping it. Once you ping the pool with a request to, say, buy a specific asset, the pool will reply with the price that it's willing to sell you that asset for. You can either accept the price and complete the transaction, or turn it down and ping again later to see if the price has moved in your direction.