  We study the response of a static Unruh-DeWitt detector outside an exotic
compact object (ECO) with a general reflective boundary condition in 3+1
dimensions. The horizonless ECO, whose boundary is extremely close to the
would-be event horizon, acts as a black hole mimicker. We find that the
response rate is notably distinct from the black hole case, even when the ECO
boundary is perfectly absorbing. For a (partially) reflective ECO boundary, we
find resonance structures in the response rate that depend on the different
locations of the ECO boundary and those of the detector. We provide a detailed
analysis in connection with the ECO's vacuum mode structure and transfer
function.
