Great question. If somebody had asked this about 3 years ago the site would have a lot less redundant information. But on the other hand, we would spend all day marking duplicates and get bored.
The answer to 'are these hidden requirements?' is no. The answer to 'buried somewhere I didn't look?' is probably yes. They are obvious requirements to practitioners and they know where to look. ECO's get special training and a manual so it's easy for them to know where to look. To an end-user, like an applicant in say, India, they wouldn't know where to look, and so these requirements might seem 'hidden'.
When you study entry clearances in a formal setting, like a law course, they give you an overview, the big picture. The 'big picture' means that the visa rules cannot exist alone in a vacuum, they have to coexist with all the other legislation Parliament makes. Plus the rules have to be fair for everyone and it wouldn't be fair to give visa applicants exemptions that native Brits don't have.
Some of the important acts that come into play when you see visa refusals are...
And a big one that your question exposes...
The Financial Services and Markets Act 2000 (money laundering, provenance of funds)
Native Brits have to abide the same rules and if something shows up in their bank accounts that appears out-of-whack, they get investigated and sometimes into serious trouble. Parliament's view is that visa applicants should not be exempted and so ECO's get trained to investigate those areas. Legal practitioners get trained when they take courses/training and so generally speaking, legal practitioners and ECO's are on the same wavelength. Coincidentally, that's why it's important to use a solicitor that's registered with the Law Society; it keeps both sides on the same wavelength.
Within the same money laundering/provenance of funds regime, they also have a list of banks they don't like. If somebody submits statements from one of those banks (or they figure out that somebody has an account at one of those banks), then the app will be refused (and they will use a different reason so that their sources are not compromised).
There is no conspiracy to keep all of these things secret. To my knowledge, it's a universal condition that countries do not make visa rules inside of a vacuum. Adding... Parliament does not make the rules in the Schengen zone, but they do not like to be wildly out-of-sync with the EEA in lots of areas. So if there's relevant legislation in the UK, there is a good chance there will be counterparts on the continent. People who are aware of the rules in one jurisdiction will have an edge in a neighboring one.
Also, UKVI is not required to list out ALL the relevant laws on their site because it would be too ponderous. They do, however, advise people to use a practitioner if their application is not straightforward. If you try to complain, they will reply that more than 90% of applications world-wide are successful.
So yes, sometimes the end-user is left having to find out these 'hidden' requirements the hard way. When in doubt, contact a lawyer.
Note: all applicants should read the guidance for specific things to include in their application.