A recent literature review shows that approximately 20–50% of energy/cost savings are possible in office buildings when accurate occupancy information is applied to the control of building energy systems. Implicit occupancy sensing, by extracting occupancy data from systems already in the building rather than from those explicitly designed to collect occupancy information, has the potential to provide high-enough accuracy for building energy management with lower costs compared to traditional explicit sensing approaches. Since more and more office workers today carry smart phones, we conducted a proof-of-concept study to explore the feasibility of using smart phone Bluetooth signals for office occupancy detection. The objective is to use existing IT infrastructure to detect occupancy to enhance building control functions while protecting office worker privacy. This paper presents some preliminary results of our recent investigation in this direction. The experimental results are very promising.