I’m not sure if this is a construction feature particular to Belém and the North in general but most apartments and houses have holes in the walls. As you can see by the photo, I don’t mean residences are old and falling apart. The idea is a pretty nice one, assuming you own a rede de dormir (hammock).
When you are done, you can either wrap it up, while still leaving it on the wall, or unhook it and close the latches on the wall. Strange, because before watching someone set one up in a mere minute, I probably would never have thought twice about what those silver things were doing on the walls.