Question you and do research on you on the open web as well as on whatever other data they can access / request. You can get selected for some form of in-depth questioning and research on you at a border for many reasons. At that point, it's certainly still possible that they miss some information about you but in times of big data collection by governments, "better" systems to make sense of the data and permissive legislation, it becomes less likely. Especially if you did not prepare in advance (for example: most people who post some hateful or threatful shit probably do not take the greatest care possible to make themselves as untraceable as possible).

I once got detained for about four hours or so at my destination airport. There were multiple rounds of questioning by persons who seemed rather intelligent and based on what they asked, they did some research about me in the meantime (or wanted to achieve that effect - no idea what tactics those people use). There could have been a few reasons why I was questioned in-depth - maybe it was because of a random spot check, maybe because they considered some stuff in my luggage unusual, maybe because I had work affiliations with people who are routinely questioned when entering some countries due to their work. Who knows.

This was many years ago, so I assume they had to do a lot of their research by hand and there would have been limited other data sources available. I imagine this is rather different today.