I think all employees can invest in the Medallion fund, which has a 50%/year return.
I suspect they use the ability to continue to invest in Medallion as an incentive to prevent people from going to work elsewhere.
Is there a data dump somewhere of Medallion's trades over time? Since the stock market is public, that means we could treat their investment engine as a black box, and try to reverse engineer their engine by looking at the decisions it makes over time. We should pay careful attention to how it performed leading up to the 2008 crash, since that reveals how their engine performs in adverse conditions, which might indicate what underlying metrics their engine is using.
I wonder what the daily trade volume of the NYSE is? It seems like if someone is moving around a billion dollars via thousands of trades, it should be possible to detect and categorize those seemingly-disparate trades as "these trades originated from the same owner".
If it's possible to trace the flow of money in that way, then it may be possible to generate a list of probable Medallion trades: search for a cohort that achieved >40% return on billions in volume.
Another way of figuring out Medallion's methods is to examine its interaction with academia. Presumably they search for candidates from fields related to their trade engine. If we can generate a list of research papers which resulted in its authors being hired by Medallion, then those research papers may reveal some of the techniques used by their trade engine.
Basically, people have thought of these strategies before. And they've thought of countermeasures to those strategies. And they probably have countermeasures to the countermeasures - for all I know, maybe RennTech's secret is that they've figured out how to statistically identify everyone in the market and trade ahead of them.
The financial industry is a massive arms race which draws some of the smartest people in the world. It's very unlikely that you (or any individual investor) can compete unless you've worked at one of the big hedge funds.
The volume traded on any exchange is absolutely massive, especially on the products where funds might be putting in millions of dollars. There's also an enormous number of prop firms, banks, institutions, hedge funds, mutual funds, and whoever else constantly buying an selling. It would be near impossible to be able to say "Firm X traded at these specific times."
Furthermore, any worthwhile firm moving that much money knows to break up its trades. If they immediately dump $10 million into the market, its going to cause a large market impact, and they're going to leak information they probably don't want to give away (since that's their edge). (See the whole Flash Boys nonsense.)
Lastly, if you could identify a set of trades that were owned by a single firm, it would be a massive benefit to anybody opposing them (ie: everybody else).
Hence they'd be just about as non-stationary as the market itself. If you were dead set on doing an almost impossible finance project, then instead of reverse engineering you'd probably be better served by figuring out how to do life cycle management of signals yourself.