At Queens Park, Stonebridge Park and Harrow/Wealdstone there are staff on the platforms to check the trains are empty before they go off into the sidings (or depot in the case of Stonebridge Park), as Bakerloo line trains are regularly scheduled to terminate at these stations throughout the day.
You could also be thinking of the security guards Network Rail pay to have along the WCML for suicide prevention, I have seen them at stations such as Kenton, as well as others on and not on the DC lines too. These people are not railway staff and are employed by security contractors such as Servo group and usually wear yellow hi-vis or other "security" type uniforms.