Obviously it depends what your goals are, but location data increasingly strikes me as among the most important. If you can track someone's location in real time, you can basically map out their entire life, including what they work on, who they meet, and why, and so on. Often cracking the "why" behind someone's life decisions is the most important.
Unfortunately many people with these datasets (often crawled without respecting robots.txt, or else obtained from a breach) are not willing to torrent the whole thing out, but provide paid APIs where you slowly search everything. (I guess if you're rich you can just purchase all of it, but otherwise it is a pain. I haven't found any easy solution yet.)
Some places you can track location
ADB flight tracker
Reverse image search across all images/videos on all social media
CCTV (on shodan/censys) with open ports or default passwords
google maps reviews, google maps links to many commercial satellite imagery sources
You can get approximate locations by many other methods - reverse phone lookup, reverse IP lookup, amazon purchase reviews, etc
You can also use indirect hits - like find out who else visited places they visit and track those people, or who met with them at the same time and track those people, or track location of people in their company, or similar.
If you are really serious about the investigation, you can hire actual people to search places they have recently visited, or stalk them irl, or similar. You can schedule interviews with and pay people in their vicinity, to get more information.
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