So, unplug the device and leave it on Alice's Desk. if Dorothy gives you a hard stare, then you know she must have had access to the video feed in the plug-in, and so is the corporate spy.
But if Dorothy instead gives Alice a hard stare, Dorothy is innocent. But if you return that morning to find the device plugged back in, Alice must be the spy.
But if Dorothy instead gives Alice a hard stare, Dorothy is innocent. But if you return that morning to find the device plugged back in, Alice must be the spy.
Unless Bob from HR got in early that morning ...