  We study the response function of Unruh-deWitt detectors placed in a flat
spacetime inside a thin matter shell. We show that the response function
distinguishes between the local and global (Minkowski) inertial frames and
picks up the presence of the shell even when the detector is switched on for a
finite time interval within which a light signal cannot travel to the shell and
back as required by a classical measurement. We also analyze how the response
of the detector depends on its location within the shell.
