It's called pig butchering. You kidnap people, hold them in Cambodia or whatever (lots of locations where local criminal gangs rule) so the locals don't bother checking for literally kidnapped slaves.
If police stop by, pay them off and make up a story about debts and punishment.
Then you use the slaves to scam others in pig butchering scams. If the slaves refuse, you beat them until they comply.
Just to clarify the terminology, pig butchering refers to the scam to steal money from fake romantic partners. The SE Asian kidnapping/slavery shops that run pig butchering scams are referred to as fraud factories.
Yeah a lot of it happens in office buildings in places like Nigeria. People wearing business casual clothes and everything. At first glance it just looks like a normal company.
So reply asking if they want you to notify authorities in their city. I doubt that the kidnappers are reading every message. And even if they are, better to let the kidnappers know that they are being encroached upon.
According to Wikipedia it seems like China arrested a few of them. It’s a fairly big concern for them since Chinese citizens are often the victims of these foreign “business opportunities”. They will warn you if you’re going to those areas.