Dell Optiplex 5440 no display (bios programming) random chat too 

You may already be doing this but make sure you record any serial numbers/unique identifiers found on the PCBs or relevant components and take plenty of pictures so you have irrefutable proof that the motherboard (in this scenario) is the same one that the customer handed over to you. I understand sometimes it can be hard to predict what a customer with bad intentions will try. Wishing you good fortune in your business. Greetings from the UK!