There should not be any “secret” or subjective records of any nature.
No there shouldn't be secret records, but that doesn't mean that you are entitled to see everything on a personnel file. There are exceptions to discloseable information; and, of course, no employer ever does anything even remotely unlawful, which is why tribunals are so very unusual!
"Subjective" records are a different matter. It could be argued that any judgement is subjective, but that does not mean it is wrong or can't be recorded on a file. One might be careful about how one records such things, but they are commonly recorded. Assessments of "behaviours", for example, are things where subjective opinion enters the fray. And gradations of subjective opinion almost always colour even "objective" matters - for example one manager may see a particular "wrong action" as a minor misdemeanour whilst another thinks it's serious, and it is entirely within reason for both to be "right" (this often comes up in disciplinary decisions).
I did mention at the weekend about being able to access the files. In a lighthearted way.My boss then dashed to the office for a while!!. You may be reading too much into this, but please be cautious. Poking a bear with a stick is never a good idea. As I said before, if you are genuinely on a zero hours contract (or have less than two years service) you can be terminated with great ease. But even people with two years employment have much less protection than they think. That untidy clearing up of yours may be nothing much to you, but the fact that it was mentioned and a picture taken suggests that somebody didn't think so, and that could very easily, with a determined manager, be your first formal warning. Being ten minutes late due to the traffic could be your second. Forgetting to put the cable back in place could be the final warning. And at that stage you can be dismissed for any minor infringement.
It's possible that the unease you have is nothing. But if it isn't, then poking the bear could be the prompt that gets you where you don't want to go. But in the end, employers do keep records, they are allowed to, and it is better to ensure that they have nothing much to record about you than it is worrying about it.