The average European is highly highly unlikely to have ever encountered an undocumented refugee from Syria or North Africa.
You also have those that are in limbo as for example there are several countries that are not taking back their citizens if they are forced there. Afghanistan used to be one of those countries for Sweden so even if someone got a no on their asylum application they could still just say no and stay. However now Sweden has a deal with Afghanistan so people who are not leaving willingly are put on chartered planes down there.
Then there are those internally within EU like beggars who are officially not allowed to stay as long as they do but no one is checking of course.
Most of Europe is very different to the US in this regard. Without being legal/registered, you’ve no chance of getting the equivalent of an SSN, drivers license, opening a bank account, leasing an apartment or in some cases even buy a mobile phone. Consequently, the number of undocumented immigrants is minuscule.
I get the impression that in the US it mightn’t be unusual for the average citizen to encounter undocumented immigrant gardeners, nannies, etc. - this almost never happens in Europe.
So it's probable that many Europeans could have encountered illegal immigrants without realising.
The common point is there a selection of low pay service jobs that attract illegal immigrants, who then become trapped. Some by debt and threats to family, and other more direct slavery. Anti-trafficking and similar groups have been warning of issues for years, and mostly ignored.
It remains a truth that the vast majority of immigration in Europe is legal, but there is a significant level of illegal migration. Thanks to the free movement of peoples within the EU, it is far harder to detect than before those border checks were pulled down.
Here is just one of a selection of pieces that have come up this week across all the press:
https://www.theguardian.com/law/2019/oct/25/trafficked-vietn...
What would be convincing would be a reference to a source with numbers/data to back up your claim.
The most recent Europe-wide figures are pretty out-of-date but indicate estimates of between 0.3% and 0.7% of people living in Eureope being irregular/undocumented (http://irregular-migration.net). This is much less than the comparable estimates for the US, for example.
Regarding the illegal immigration situation in EU is very different from US because the laws are different. In EU state helps immigrants so they're incentivized to register, and also it's easier to get permits. According to the official DHS report [2]: "Of the total illegal alien population in 2015, nearly 80 percent had resided within the United States for more than 10 years", which is super uncommon in EU, they'd be legalized by that time.
So just comparing the numbers doesn't tell the true story, but according to the official report from Austria [3]: "Irregular residents account for between 2.9% and 1.1% of the total population.", so 10x more what you suggested. They also note in this report: "As there are strong incentives for immigrants to register in Austria (e.g. certificate of registration as a prerequisite for application for state benefits), the majority of immigrants are on record at the registration offices. In contrast, very little information is available about irregular immigration." And Austria is one of the stricter countries in this regard.
[1] https://ec.europa.eu/eurostat/statistics-explained/pdfscache...
[2] https://www.dhs.gov/sites/default/files/publications/18_1214...
[3] https://bmi.gv.at/Downloads/files/Bericht_des_Migrationsrats...
1- For face f, is f a member of F_known? (From national ID cards etc.) Assume this gives an answer [0.0 - 1.0].
2- Is f a member of F_suspect? Where F_suspect is a database of face imagery associated with potentially illegal activity or refugee migration. Not that these faces could be from video surveillance (from a border region, train station, migrant camp) or scraped from war zone images e.g. jihadi propaganda, YouTube video of regime critics, talking politics on HN, video of protests in other countries, etc.
3- Not matching anything, and hence also requiring investigation.
All of these have errors, which generally go up as more people are added to the training database. Apparently some ethnicities (okay, black people) are often trained for poorly. This will likely result in 'over-servicing' of some ethnicities.
If local police get a hit for someone going through festival security and detain them on the basis of face recognition software, what is the legal basis for that? Police would say that they had 'intelligence' but can they explain the basis for it? Is this not essentially maintaining dossiers on people who have not committed any crime? They have no evidence a crime has been committed but instead they are essentially running a police checkpoint and saying "Papers!". These are the mechanisms of the Stasi.
One can argue that police should be able to detain those with outstanding warrants. I have no problem with that, except that when police take photos of citizens it is hard to know what is done with those photos. Ironclad legislation requiring they not leave the device or be stored on anything except ephemeral storage would be a good start, along with civil and criminal penalties for non compliance and direct individual responsibility and penalties for the chain of command.
Without a rigorous push back, America will become like China and the UK, where your face must be shown, and it will be recorded and tracked 24/7. Of course in America it will probably be via a company, and police forces will have a subscription, which lowers their fees the more faces the capture.