Read Missing stair (Wikipedia)

Missing stair is a metaphor for a person within a social group who many people know is untrustworthy or otherwise has to be "managed", but who they work around by trying to quietly warn others rather than deal with openly. The reference is to a dangerous structural fault such as a missing stair in a home, which residents have become used to and accepting of, and which is not fixed or signposted, but which (most) newcomers are warned about.