I've recently had the pleasure of doing a short presentation on OSINT aka "Open-Source Intelligence". Mind now, I am no professional in this matter and the following is a (very) simple introduction to the matter from the PoV of an amateur.
OSINT, to put it simply is the use of publicly available information to find answers to your questions. In this regard I think it's fair to say that everyone has done this at some point in their lives—the basics of it aren't exactly rocket science. However. When it comes to the more advanced, techical parts of OSINT - it starts to appear quite magical indeed, the ability to create intelligence from what the average viewer would consider nothing at all.
I believe that most people, while aware of what it means to post something online, aren't quite aware of "how deep the rabbit hole goes". Even the simple fact of Internet archives like the Wayback Machine will archive webpages, meaning that even if a site is deleted, it will most likely remain accessible through archives; once something is posted online, it's there forever. Of course this isn't a 100% absolute but should, in my opinion, be taken as a guideline of online interaction.
To bring a case of Hansel and Gretel's trail of breadcrumbs as it applies to searching through open-source information, I'll use myself as an example. If you were to search for my full name (not exactly a secret in most cases) on Google, you would simply find some other individuals with similar names, a result of Google taking your search input and taking whatever bits and pieces of it that it likes. If however you add quotation marks, Google will search for that exact phrase—in which case you will in fact find a few hits about me - an archived local newspaper article of 1st year primary school students enrolling in and a case of 9th year students graduating the local schools. And while searching for my name doesn't give many results, this search has now provided you with the name of the school I went to and the names of everyone that I went to school with—people who I know and maybe just maybe have a connection with on some social media site. As it turns out, even if looking directly for me doesn't give any results, I can still be found through the social media of others - there I am, added as a friend on the profile of someone I went to school with, not with my full name but enough of it that, with context provided, it can be confirmed to be my profile page - from which you could get additional information a la a username—another piece in the trail of breadcrumbs. Breadcrumbs, which, if followed could lead to even more information: secondary usernames, emails, passwords (through data breach archives), etc. I won't go further with this example as I believe it has illustrated the point I'm trying to make - even someone who has a minimal online presence can have enough of a footprint to be identifiable through publicly available information.
OSINT of course goes far far beyond what I've described here, some cases of journalism through OSINT for example really highlight just how much information is available if you know how to look for it. In this case I would highlight Bellingcat and the example of MH17 for further reading.
P.S. Do check if your information is out there in data breaches. A few sites to help with that: HaveIBeenPwned & Intelligence X.
And OSINT Framwork as a general set of OSINT tools and websites to play around with, so you can be aware of at least some of the possibilities when it comes to OSINT
- Stern Kittel