I imagine this very well being extended not just to match photos, but also metadata within the photos. And then the list of things it matches against is extended from CP to other undesirables by society.

After a 12 hour flight - that was of course delayed - Liam was pretty exhausted, but was looking forward to getting to his hotel in the center of Munich. He got to the front of the queue, and handed his passport over to the customs officer. The officer scanned Liam's passport, took it off the reader, and after 20 seconds asked "You flew from Los Angeles today?". Liam replied "Yes...". The customs officer, with his firm German accent, said "I need to check something with my colleague, wait here.". Not that there was anywhere Liam would go.

The customs officer came back with someone else who was slightly older and clearly more senior. The senior officer said "Come with me please", and led Liam to a room at the side of the customs hall. The officer said "Sit down please", indicating to the chair in front of the desk. The room looked like any other office, with a computer on a desk and chairs either side. The officer sat behind the desk and started typing something on the computer. After a few minutes he said "You are wanted by Interpol".

The customs officer explained to Liam that he had been flagged as a photo he had taken 6 months before included a known terrorist, and so by association Liam had been flagged. Liam asked how they accessed his photos - he is tech savy and only takes encrypted backups onto his own devices. The customs officer explained that they didn't need to, as this flagging had been done entirely by his device. The customs officer gave the date of the photo, and Liam found it on his device. He had been on holiday with his girlfriend in Paris, and they had taken a selfie. There was someone clearly visible behind them, and the customs officer explained that the facial recognition had identified this person. Due to privacy laws he wasn't able to say (or even see themselves) who this person was, only that they were on the highest German terrorist watch list.

From the photo it looked quite obvious that this person was just a passer by who glanced at the couple just at the moment they were taking a photo. The customs officer took Liam's fingerprints and asked Liam questions about his trip to Paris - typing the answers into the computer - and then the computer decided that Liam could be released. However the customs officer told Liam that he would be closely monitored while in Germany, and may receive 'check in' calls from police. He told Liam he must answer them otherwise a team will be dispatched to intercept him. Liam was then allowed to go on his way. He was only delayed by 45 minutes, but it wasn't a great start to his holiday in Germany. 3 years later when he visited Germany again the same thing happened, at least he knew what to expect this time...

(This is partially based on something that actually happened to me. Nearly a decade ago my passport was stolen, and every time I go to Germany I need to have a fun conversation with customs officers. Every other country I've visited - including the US - let's me through without even mentioning it)