  The multi-sector intelligent surface (IS), benefiting from a smarter wave
manipulation capability, has been shown to enhance channel gain and offer
full-space coverage in communications. However, the benefits of multi-sector IS
in wireless sensing remain unexplored. This paper introduces the application of
multi-sector IS for wireless sensing/localization. Specifically, we propose a
new self-sensing system, where an active source controller uses the
multi-sector IS geometry to reflect/scatter the emitted signals towards the
entire space, thereby achieving full-space coverage for wireless sensing.
Additionally, dedicated sensors are installed aligned with the IS elements at
each sector, which collect echo signals from the target and cooperate to sense
the target angle. In this context, we develop a maximum likelihood estimator of
the target angle for the proposed multi-sector IS self-sensing system, along
with the corresponding theoretical limits defined by the Cram\'er-Rao Bound.
The analysis reveals that the advantages of the multi-sector IS self-sensing
system stem from two aspects: enhancing the probing power on targets (thereby
improving power efficiency) and increasing the rate of target angle (thereby
enhancing the transceiver's sensitivity to target angles). Finally, our
analysis and simulations confirm that the multi-sector IS self-sensing system,
particularly the 4-sector architecture, achieves full-space sensing capability
beyond the single-sector IS configuration. Furthermore, similarly to
communications, employing directive antenna patterns on each sector's IS
elements and sensors significantly enhances sensing capabilities. This
enhancement originates from both aspects of improved power efficiency and
target angle sensitivity, with the former also being observed in communications
while the latter being unique in sensing.
