  Recent research on the development of a thermal cloak has concentrated on
engineering an inhomogeneous thermal conductivity and homogeneous volumetric
heat capacity. While the perfect cloak of inhomogeneous $\kappa$ and $\rho c_p$
is known to be exact (no signals scattering or penetrating to the cloak's
interior), no such analysis has been considered for this case. Using analytic,
computational, and experimental techniques, we demonstrate that these
approximate cloaks are detectable. Although they work as perfect cloaks in the
steady-state, their transient (time-dependent) response is imperfect and a
detectable amount of heat is scattered. This is sufficient to determine the
presence of a cloak and any heat source it contains, but the material
composition hidden within the cloak is not detectable in practice.
