The escape of accelerated particles represents a fundamental piece of information in the context of the supernova remnant (SNR) paradigm for the origin of Galactic cosmic rays (CRs) to interpret both the observed CR spectrum and the radiative signatures emerging from these sources. The particle release process from SNR shocks is energy dependent, such that high-energy particles are expected to leave the shock region before low-energy ones, the absolute temporal scale depending on the diffusion coefficient in the region surrounding the shock. This fact might result into broken power law spectra of hadronic gamma rays, similar to those observed from several middle-aged SNRs. The presence of dense gas targets, such as interstellar clouds, nearby the accelerators might provide evidence for the escaping flux of particles through the production of gamma rays and neutrinos at hadronic collisions. Exploring such scenarios is extremely timely given that recent wide field-of-view survey instruments have revealed an unexpectedly large population of Galactic gamma-ray sources at ultra-high-energies (UHE, E >100 TeV) with no counterpart.