Either way, this sort of thing is against the law in many countries. For instance, where I live you have to have a permit otherwise it is a pretty serious crime.
Because when you're research individuals you are crossing the line between 'just futzing around' and 'building up a file on a person without their consent'.
Is there specific legal guidance about whether doing this in a training run is against the law? It seems like your link is for commercial / for-hire investigative work.
Who would build up files like this in a private context?
The link you've given doesn't have any relevance to this, that's a page for registering a private security agency. I can assume you hallucinated the first answer and are now just linking random sources.
Reading comprehension not your strong suit?
That's doxxing. The gathering part is illegal in some countries, usually with a ton of caveats like doing that with malicious intent. Illegal or not, it raises some questions.
Isn’t it only doxxing when you publish the information?