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I'm staying in an apartment in Mexico and found this mysterious shower drain cover/cap/lid in the bathroom, made out of plastic:
Is it to prevent insects from crawling out? To prevent bad smells from leaching? To decoratively hide the shower drain? What's the name of this "device" and how is it supposed to be used?
The device is a universal drain stopper. It is designed to keep water in a bathtub or sink. Because it works by sitting on top of the drain, it is not necessary for the consumer to worry about the diameter of the drain opening, as is the case with traditional drain plugs that sit inside the opening.
These work by creating a seal against the smooth surface of the tub or sink around the perimeter of the drain, being held there by the weight of the water. They are not effective at keeping gases from escaping, therefore, unless the basin is full of water. Furthermore, they are not particularly effective with drains such as the one in the photograph, because of of the joints between the tiles. The material is not flexible enough to seal these joints.
The purpose for which this thing has been placed on this drain, therefore, is unlikely to be related to sewer gases, because it will not be very effective for that purpose. It might be marginally more effective at keeping water out of the drain, but probably only for a couple of minutes, so that is also unlikely to be the reason.
The speculation that it is to keep scorpions or vermin of a similar size from escaping the drain seems more likely. This would be effective against creatures that are able to swim through the water in the trap, small enough to pass through the metal drain cover, too large to crawl through the tile joints, and not strong enough to move the cover. If there are any local scorpions or other vermin that have these characteristics (cockroaches?), that's probably your answer.
Have you tried asking the owner of the apartment or other people who live in the area?
Having lived in Texas, I would use that to keep out scorpions, which can crawl up drains and even survive the water coming down them. Though I'd also change the strainer to a different design that a scorpion could not so easily squeeze through...
My guess is that this would be to prevent sewage smell coming into your room. It's a common problem, see for example this Q&A online.
While I haven't seen such lids before, I've come across many such drains which allowed sewage odors into the hotel room. This was especially a concern in the following conditions:
A well sealed-off room, common with newer hotels in hotter countries to prevent hot air coming in while airconditioners are running.
A powerful bathroom exhaust fan, meant to let out steam after a hot shower to combat mould growth.
A lack of a proper siphon in the shower drain. I am not sure why they don't always make them properly, though it could be because the floors aren't thick enough to allow for high enough siphons.
The lid might be an attempt at closing off the drain so gasses cannot pass though, though I'm not sure if it provides enough of a seal to work.
