  The LHCb detector provides accurate vertex reconstruction and hadronic
particle identification, which make the experiment an ideal place to look for
light long-lived particles (LLP) decaying into Standard Model (SM) hadrons. In
contrast with the typical search strategy relying on energetic jets and a high
multiplicity of tracks from the LLP decay, LHCb can identify LLPs in exclusive,
specific hadronic final states. To illustrate the idea, we study the
sensitivity of LHCb to an exotic Higgs decay $h\to SS$, followed by the
displaced decay of GeV-scale scalars into charged kaons $S\to K^+K^-$. We show
that the reconstruction of kaon vertices in narrow invariant mass windows can
efficiently eliminate the combinatorial backgrounds from $B$-meson decays.
While the same signal is extremely difficult to probe in the existing displaced
jet searches at ATLAS/CMS, the LHCb search we propose can probe the branching
ratio BR$(h\to SS)$ down to $0.1\%$ ($0.02\%$) level with $15$ ($300$)
fb$^{-1}$ of data. We also apply this projected bound to two scenarios with
Higgs portal couplings, where the scalar mediator $S$ either couples to a) the
SM quarks only, or b) to both quarks and leptons in the minimal flavor
violation paradigm. In both scenarios we compare the reach of our proposed
search with the expected constraints from ATLAS and CMS on the invisible Higgs
width and with the constraints from rare B-decays studies at LHCb. We find that
for 1 GeV $< m_S < $ 2 GeV and $0.5~{\rm mm} \lesssim c \tau \lesssim 10$ mm
our proposed search will be competitive with the ATLAS and CMS projections,
while at the same time providing crucial information of the hadronic
interactions of $S$, which can not be obtained from the {\it indirect}
measurement of the Higgs invisible width.
