The threshold will differ, because the requirements vary. In some places, lectures are not mandatory, and so no, no one would necessarily check. In others, you're allowed a certain percentage of absences. And in others, it's compulsory, and a register is taken, and people would check much more quickly.
It is a real problem. If you've a student studying something lab-based or with lots of contact hours, it may very quickly become clear that things are not right and they're not showing up. OTOH if a student only has perhaps two fixed things they must attend each week - or if it's their dissertation term and they have even fewer, or if it's reading week - it can be much easier for things to go unnoticed.
The problem is, too, that if you're in charge of a year group of 100 students, in any given week perhaps 20-40 of them will miss things. Almost always, all of them will have mundane reasons for not turning up - they've caught flu, they slept late, they didn't write the essay so they decided not to show, etc. etc. It wouldn't be sensible or fair to chase every single time (because you're trying to teach them to be responsible adults, not school children).