The spell caster chooses a door he can see (and must remain in physical contact with it for the entire casting time). When the spell is casted, the door completely disappears of sight (becomes invisible) and its handle ceases existing. To open it, one must simply touch the middle of the structure, but that can be a hard thing to do considering that the person probably doesn't even know that the door exist.
The spell caster and anyone he chooses while casting the spell have the ability to recognize the hidden passage instantly, but no one else can without using Detect Magic, Truesight or other spells/abilities that the GM finds apropriate.
Still, anyone can touch a random spot on a wall and accidently opening a hidden passage, but that should be really hard to happen.