📚 In need of PC real-time incognito screen capturing software

I do freelance PC work for a local "Pack and Ship" type shop. The shipping stations are all Windows machines. The owner suspects one of it's employees is skimming the till during transactions (not sure which monetary type). He has CCTV placed throughout the shop and the employees know the cameras are present. He cannot get a clear shot of what is going on at the screens, since they are standing in front of the screens. He asked me about some screen capturing software that runs in the background without the user knowing about it. It would be ideal if something could be set to run on a schedule, but if not, that would be fine. Your thoughts/suggestions, please? Thank you