Notes of a Native-Speaking English Teacher

It's not easy to make sense of it, but it turns out that this is a request to inspect the gas pipes inside the apartment.  Apparently, since most Korean households have gas service and because people live so close together, the gas company does regular inspections.  

The problem is that they come without prior notice to perform a non-emergency service during extremely late hours.  

Even with the note on the door, there's typically no hint at when they will return.  Sometimes it's the following day, sometimes it's 3 days later.