  In this paper, we consider target detection in suspicious tissue via
diffusive molecular communications (MCs). If a target is present, it
continuously and with a constant rate secretes molecules of a specific type,
so-called biomarkers, into the medium, which are symptomatic for the presence
of the target. Detection of these biomarkers is challenging since due to the
diffusion and degradation, the biomarkers are only detectable in the vicinity
of the target. In addition, the exact location of the target within the tissue
is not known. In this paper, we propose to distribute several reactive
nanosensors (NSs) across the tissue such that at least some of them are
expected to come in contact with biomarkers, which cause them to become
activated. Upon activation, an NS releases a certain number of molecules of a
secondary type into the medium to alert a fusion center (FC), where the final
decision regarding the presence of the target is made. In particular, we
consider a composite hypothesis testing framework where it is assumed that the
location of the target and the biomarker secretion rate are unknown, whereas
the locations of the NSs are known. We derive the uniformly most powerful (UMP)
test for the detection at the NSs. For the final decision at the FC, we show
that the UMP test does not exist. Hence, we derive a genie-aided detector as an
upper bound on performance. We then propose two sub-optimal detectors and
evaluate their performance via simulations
