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.
https://youtu.be/dU9hrd35Dsg?si=jkh8JPFc6C4ooP8Z