Using the directional antenna to locate the target is suitable to use in a large free open space. It is not a good idea for indoor use.
RF signal has multiple reflections in a closed environment especially in a building.
Signal comes from all directions making a directional antenna not functioning as expected.In addition, using a directional antenna will miss most of the signals except it is coming from the direction the antenna is pointing to.
RFD-10 4G LTE Cell Phone Detector is designed to catch short message transmission. A directional antenna will render its performance.