When I moved into my apartment, I noticed a long, recessed box in the bathroom wall. I believed it was something awkward such as for soap or old razors. It felt old and out of place.
After some investigation online, I discovered it’s a built-in tissue box holder. These are popular in mid-century homes and designed to hold half-sized facial tissue boxes. A built-in tissue box holder, being recessed, usually right next to the sink, kept tissues accessible from a convenient location without occupying counter space.
Typically in ceramic or metal to correspond to bathroom tile, instead of having a tissue box on the counter, mid-century homeowners used a built-in tissue box holder for keeping tissues, a clean, simple solution, while also providing a subtle method to allow bathroom tissues an element of design.
Most people today confuse these for old soap holders or hearse-style blade disposal slots. If you just look at them without the original contents you might think that they are completely random—especially if the tissue box is missing from the built-in tissue box holder. However, if the shape is correct, and it was not very close to the toilet, your best luck is a tissue box.