On the other hand if you're entering a car in the Demolition Derby you don't enter the most expensive car you own. We could be looking at the chassis he wears for the fighting ring because when it gets broken the parts and repairs are inexpensive. Even if it's not powerful/fast enough to make much prize money, that's all right if the prize money it makes can keep it repaired.
He may have another chassis that looks like a puppy, or a gothic-lolita doll two feet tall modeled after Rozen Maiden. Or, you know, he might live on a server downtown, spends his weekdays optimizing utility usage for city buildings, and this is how he goes out on the town on weekends.
As for the robot fighting ring being "secret", that's crock. No place that relies on customers being present can be secret from the customers. No coordinated action that recruits people can be secret from the population they're recruiting from. And the police are members of the same populations. QED. That's sort of like the idiot BART protesters last year who couldn't figure out how the cops always knew where they were going to be - of course they were coordinating the whole deal on social media and that was all public, but they were still convinced the cops couldn't know unless somebody was ratting them out.
In most cities across the US, prostitution is illegal. But a traveler in an unfamiliar city can pick up a darn phone book and immediately identify places to call for a temporary partner of negotiable affection. Seriously, just look! If actual prostitution is illegal there, they'll be under some easily-identifable misclassification like 'massage' or something, and while the adverts won't actually say sex-for-money, they'll use pretty much every euphemism short of that. Are the cops actually dumber than the average traveling salesman, or would busting the hookers just interfere with their profits?
And this is why I think any law which isn't 100% enforced at every opportunity should be repealed instead. If something is neither clearly legal nor illegal with quick and effective enforcement, it merely promotes government corruption because the agents of the government will be able to shake it down for a cut of the profits in exchange for failing to do their jobs.
Underground fighting ring? Same deal. Customers know where to come, competitors know where to come, QED Cops know it too. Not being busted means there are cops on the take, just like with prostitution.