"C" meant "Concealed Door".
Similar to a secret door, but not as hard to find.
Whereas a secret door was purposely built, and usually have some secret catch that you need to find in order to open, a concealed door is just obstructed in some way (debris, old painting leaning against the wall, etc).
Usually, you can find it with a relatively easy spot check once you're near or actively looking for secret doors.