According to Wikipedia:
In economics and business decision-making, a sunk cost is a cost that has already been incurred and cannot be recovered. Sunk costs are contrasted with prospective costs, which are future costs that may be avoided if action is taken.
Meaning, the company already either owns the building or is leasing it. That's going to be a fixed price whether employees come in or not, so what are they gaining financially by this? (Assuming control and employee trust is no the main concern.)