Internal Security Department

The Internal Security Department is a secret police force and intelligence agency of the SCP Foundation, which is responsible for guarding the organization's information security by investigating potential moles and traitors serving rival Groups of Interest. Due to their officially classified status, they are commonly regarded as just a myth. The Internal Security Department Also oversees the ethics committee in some cases as they they may have to arrest ethics personnel and internal security will arrest Corrupt personnel charged with Corruption, Treason, Desertion 
Add on: The Internal security department also shows up in areas like the S3 Checkpoint where they guard personnel who are not allowed in that area.