Belmar’s only regret at that time was not insisting the camera come down or be turned off unless a suspect was inside the room, he said.

“It’s highly unusual for a third party to monitor police officers,” said Belmar, who argues the cameras are violations of county officers’ privacy.

On Monday, the substation at North Hanley Road where officers were videotaped covering up a security camera was empty. Fragments of green tape remained stuck to the sides of the camera — the same kind of tape normally used to seal evidence.

Bi-State installed the camera after a county officer struck a man’s hand with his baton, breaking three of the man’s fingers, and then held him in the substation for 90 minutes in 2014.

A sign on the wall reads: “You are being recorded.” A computer displayed footage from security cameras on the platform.

Belmar has said his officers use the room to change clothes, but the door to the substation is not lockable from the inside. The office is near the train platform, and hundreds of riders pass it walking to and from a commuter parking lot. Any one of them could open the door.

About 25 steps from the substation is a private restroom to which Bi-State employees and county police have access.