After nearly 40 years as a crime investigator in various roles I have just completed an (expensive) course which assists me to find lots of information about people that they think is confidential.
All information I uncover is on the open internet and is legal. I will be using this knowledge for good purposes. For example, when you put messages and photographs on Facebook it goes to your friends and it is unethical and illegal to gather that information. The problem is when your friends share that information it could well go into the ‘open’ net. ‘Good luck on your house move next Monday’ (That information is worth the expense of a surveillance team).
This is just a snippet, there are numerous free sites and some I have to pay for. There is so much out there, house purchase, home improvements, ancestry, LinkedIn, Twitter all such sites help build the picture.
Only those that have been up to no good need be concerned that investigators are being hired to trace them.