  An analysis is presented of the behaviour of muons in the low-temperature
state in spin ice. It is shown in detail how the behavior observed in some
previous muon experiments on spin ice in a weak transverse field may result
from the macroscopic stray field of magnetized spin ice. A model is presented
which allows these macroscopic field effects to be simulated and the results
agree with experiment. The persistent spin dynamics at low temperature
originate from the sample and could be a muon-induced implantation effect that
is operative in out-of-equilibrium systems with long relaxation times.
