In this paper, we present the mathematical analysis of discovery performance in device-to-device (D2D) communication system. As opposed to the previous work which deals with outdoor devices only, we consider a non-homogeneous environment in which both indoor and outdoor devices exist with the different intensities and channel characteristics. A stochastic geometry model with various system parameters, including inband emission and channel parameters, has been employed to derive the average number of devices discovered within the given time constraint. It has been found from our analytical results that the in-band emission is a most critical factor that limits the discovery performance, especially in the mixed environment, implying that its performance can be improved significantly by any means of reducing the in-band emission.