A worker who is usually exempt and working project hours which can be any time during the day or evening. They often live at their desk, have minifridges and coffee makers there. They disappear often during the day without being noticed and come back to work late hours. Their job is very solitary and they are so seldom seen that seeing them is like seeing a phantom.
The opposite of a desk phantom is a desk mannequin who is usually more visible, and more customer facing.
We just saw John coming in at 2 with coffee and his van was in the parking lot all last night. We think he's working on the bank acquisition code but you'll have to ask Francine his boss about that. The guy is a real desk phantom.