There may exist a collection of particles, which have different dynamics from SM particles and interact with SM particles only feebly through a portal particle, e.g., dark photon. Such an idea, called a dark sector, has been attracting growing interests, since dark matter may reside in a dark sector. Intensity/lifetime frontier experiments search for visible decay of long-lived dark sector particles. Specific plans are searches with forward (FASER) and off-axis (MATHUSLA) detectors at high-luminosity LHC, and with a new detector (DarkQuest) at the Fermilab SeaQuest experiment. They may provide an intriguing possibility to probe a GeV-scale dark confining dynamics. Some of associated dark hadrons are good dark matter candidates, e.g., strongly interacting massive particles and asymmetric dark matter, while others decay into SM particles with a long lifetime. We discuss the sensitivity of the near-future intensity/lifetime frontier experiments to dark hadrons.