  This Letter proposes a new signature for confining dark sectors at the LHC.
Under the assumption of a QCD-like hidden sector, hadronic jets containing
stable dark bound states originating from hidden strong dynamics, known as
semi-visible jets, could manifest in proton-proton collisions. In the proposed
simplified model, a heavy $Z'$ mediator coupling to SM quarks allows the
resonant production of dark quarks, subsequently hadronizing in stable and
unstable dark bound states. The unstable dark bound states can then decay back
to SM quarks via the same $Z'$ portal or photons via a lighter pseudo-scalar
portal (such as an axion-like particle). This mechanism creates a new signature
where semi-visible jets are enriched in non-isolated photons. We show that
these exotic jets evade the phase space probed by current LHC searches
exploiting jets or photons due to the expected high jet neutral electromagnetic
fraction and photons candidates non-isolation, respectively. In the proposed
analysis strategy to tackle such signature, we exploit jets as final state
objects to represent the underlying QCD-like hidden sector. We show that, by
removing any selection on the neutral electromagnetic fraction from the jet
identification criteria, higher signal efficiency can be reached. To enhance
the signal-to-background discrimination, we train a deep neural network as a
jet tagger that exploits differences in the substructure of signal and
background jets. We estimate that with the available triggers for Run 2 and
this new strategy, a high mass search can claim a $5 \sigma$ discovery
(exclusion) of the $Z'$ boson with a mass up to 5 TeV (5 TeV) with the full Run
2 data of the LHC when the fraction of unstable dark hadrons decaying to
photons pairs is around 30 %, and with a coupling of the $Z'$ to SM quarks of
0.25.
