So do you know what a PI is generally hired for?
There's a narrow swatch of misbehavior that the "skeletons" would need to be, for a PI to take the case and get involved. The client would have to have some vested interest or harm done to them, or some idea that the target is doing some harm to somebody before taking the case.
If the idea was get dirt to be vindictive, the PI would not take the case.
If they did take the case and there was evidence that things were clearly criminal, (quid pro quo, malfeasance, etc) they would refer the case to actual police. They would only continue investigating if the police declined to investigate, and their purpose would be privately prosecuting the person. ( Basically filing a suit to whatever court, like you were suing the person but you prove the criminal acts and they get sentenced potentially)
If the skeleton was more of a civil harm like a breach of duty or breach of contract, the PI would gather evidence relevant to the harm, and not provide their client with irrelevant information like who their favorite callgirl is or whatever bathhouse they frequent. They would also not share info about infidelity unless their client was the spouse that was being cheated on.
Still, what you think a legitimate reason could or couldn't be probably doesn't match up with what actually would be the basis of some surveillance.
This comment is super cliff notes, and based on some PI training in Ontario Canada that I couldn't make myself complete after I realised that it would be more of the same bullshit shiftwork that I was trying to get away from 10 years ago.
OP might have a good time calling some PI firms local to them and asking to pay for a consult and fact check his narrative.
Legitimate reasons could be contrived. The question isn't what it takes to get a PI to investigate, the question should be what it would take for them to spill unrelated tea.
The first step in opening an investigation is investigate your client. Their relationship to the subject and the validity of the harms that they might have suffered or the validity of whatever narrative of criminality.
If you manage to pass that smell test, the information you get from the investigation would be severely limited to the scope of validating and proving that the harms occurred.
Again, you could get them to do stuff for you. You might not be able to get any useful information past that.