It's just a gameplay mechanism. If you want actual realism, then every 'day' in the game should take 12 or so hours to complete, you should have a meal break and regimented toilet breaks, so on and so forth.
I mean, no-one believes that a border crossing is only open for a few minutes only per day, any more than they believe there's an all-powerful control after yours. It's like how the Sims put in toilet requirements for simulation purposes, but it turns out that your character can go without the toilet for a couple of days, then miss half a shift at work because of a visit.
Immigrant communities often live together. Let one known undocumented worker in, and State Security can then follow them to uncover existing communities.
That part actually bothered me, to be frank. For several reasons.
1. It indeed gives the impression that your work is monitored, which should not be the case.
2. It's a mechanism to give you penalties, but it does not really affect the story-line in itself. It would have been great if a person you let pass by mistake actually ends up changing the situation (i.e. becoming a terrorist the next day or something) and therefore introducing more regulations to make your work more difficult.
3. It could have been done in a more subtle way. For example, the guards at the right of the screen could do a random check on top of the ones you do, and find out, not 100% of time but at a certain ratio, you actually make/made mistakes.
Eh, it's somewhat reasonable to expect the failures to get caught quickly. Perhaps there's routine immigrant checks at housing estates, or an employer finds out and reports the illegal immigrations.
It's like there's an invisible control just after yours that never misses.