Which would save HR doing it and sending a memo about health and safety and asthma can kill due to these, possible.... Yeah, that is exactly how that would play out in many companies. At least in the UK.
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.