I came across this sign in a quite unique, hard to describe shopping mall called Kasvihuoneilmiö in southern Finland. One of the very many different things in that mall is a mirror room where all the interior walls, the ceiling and a large part of the floor is covered with mirrors, creating a cool visual effect. What I assume are stylistic reasons, also the exterior door of that room is a mirror, and made to look like a large, elaborately framed wall mirror.
The said door naturally has a handle to pull, which is what I took a picture of because of the small note saying "Avaa ovi!" (lit. Open the door!) above it. This caught my attention as usually door handles do not have explanatory signs to tell that the door is openable.1 But here a sign is necessary because it is not obvious that we are looking at a door! Doors are quite rarely mirrors with ornate frames, so even with the door handle providing the affordance of being operable as a door, the average passerby might not understand to try and yank or push the handle.
Because of the handle, this is not an intentionally hidden door, but it is one that is effectively hidden as the door knob itself has clearly not been enough to identify this non-standard door as a door.
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Sometimes they are needed though, especially when the "pull or push?" distinction is not clear - such badly designed doors are sometimes called "Norman doors" after Donald A. Norman, the author of the beautiful read Design of Everyday Things ↩