Photon emission microscopy (PEM) provides a valuable first step in the failure analysis process. An analysis of a mixed signal bipolar/CMOS silicon on insulator (SOI) device revealed an abnormal emission site that appeared to emanate from the oxide isolation ring. Subsequent mechanical probing of the emitting bipolar transistor revealed node voltages nearly identical to a known good reference unit that had no emission site at the affected transistor. This article analyzes the reasons for the emission site on one transistor and not the other even though the node voltages were the same. It was observed that while the node voltages were nearly identical, the available current paths were not. The different paths directly related to the amount of available carriers for recombination in the base. The construction of the SOI device creates unique optical paths for emission sites not observed in non-SOI devices. It can be concluded that the failure mechanism does not always reside at the abnormal PEM site.