  We explore the discovery potential for long-lived particles at the 250-GeV
ILC. The goal is to investigate possible gains of a dedicated far detector over
the main detector ILD. For concreteness, we perform our study for sub-GeV
axion-like particles $a$ produced via $e^+e^- \to a \gamma$ or $e^+e^- \to Z
\gamma \to (a\gamma)\gamma$ and decaying into pairs of charged leptons. In the
ideal case of zero background and perfect detection efficiency, we find that
far detectors placed in the planned underground cavities or a large cuboid on
the ground can enhance the sensitivity to long-lived pseudo-scalars at best
moderately. On the other hand, the ILD itself is a perfect environment to
search for long-lived particles, due to its excellent angular coverage and
radial thickness. For long-lived particles produced with cross sections of a
few picobarns, the ILD could probe lifetimes up to 300$\,$ns, or proper decay
lengths up to 100$\,$m, in 250$\,$fb$^{-1}$ of data. For axion-like particles
produced through weak interactions, the ILC can reach an even higher
sensitivity than searches for displaced vertices in meson decays at Belle II.
Our findings apply similarly to other proposed electron-positron experiments
with a high angular coverage, such as the FCC-ee and CEPC.
