In 2005 we carried out muon spin relaxation (muSR) experiments in Pr(Os1-xRux)4Sb12 alloys at KEK, Tsukuba, Japan, to investigate the time-reversal symmetry breaking found in the end compound PrOs4Sb12. We observed a rapid suppression with Ru doping of the spontaneous local field Hspon that indicates TRS breaking in PrOs4Sb12. To our surprise, similar experiments at ISIS in 2006 yielded an increase in Hspon below Tc in Ru-doped samples, in disagreement with the KEK results. A possible source of this discrepancy is a difference in defect structure, which can generate a spontaneous field in TRS breaking superconductors by creating inhomogeneity in the superconducting order parameter. We propose zero- and longitudinal-field muSR measurements in unannealed and annealed Ru-doped Pr(Os1-xRux)4Sb12, to determine the dependence of Hspon on defect density and hence shed light on its origin.