  The dynamics of a particle in an expanding cavity is investigated in the
Klein-Gordon framework in a regime in which the single particle picture remains
valid. The cavity expansion represents a time-dependent boundary condition for
the relativistic wavefunction. We show that this expansion induces a non-local
effect on the current density throughout the cavity. Our results indicate that
a relativistic treatment still contains apparently spurious effects
traditionally associated with the unbounded velocities inherent to
non-relativistic solutions obtained from the Schroedinger equation. Possible
reasons for this behaviour are discussed.
