Accurate ranging and localization are crucial applications for wireless sensor networks. Reliable position information, obtained with a reasonable complexity overhead and possibly low energy consumption, is necessary for a successful operation of these networks. The paper regards the analysis of the performance bounds of an approach for improving the trade-off between energy consumption and performance in a localization and tracking process. A set of fixed beacons is used for localizing a target that is moving in a limited environment and tracking is realized by mixing active signal transmissions, which allow using standard techniques for deriving distances and locations, as well as passive signal receptions, which exploit scattering caused by the target during signal propagation. Obviously, when the beacons exploit passive signal reflections, the target does not consume energy, differently from the steps characterized by the active exchange of radio signals. In the paper we present the analysis of the posterior Cramer-Rao Bound of this technique for a general scenario, highlighting the trade-offs between the energy savings and the performance costs in terms of localization error.