  In analyses of target detection with Gaussian state transmitters in a thermal
background, the thermal occupation is taken to depend on the target
reflectivity in a way which simplifies the analysis of the symmetric quantum
hypothesis testing problem. However, this assumption precludes comparison of
target detection performance between an arbitrary transmitter and a vacuum
state transmitter, i.e., ``detection without illumination'', which is relevant
in a bright thermal background because a target can be detected by its optical
shadow or some other perturbation of the background. Using a target-agnostic
thermal environment leads to the result that the oft-claimed 6 dB possible
reduction in the quantum Chernoff exponent for a two-mode squeezed vacuum
transmitter over a coherent state transmitter in high-occupation thermal
background is an unachievable limiting value, only occurring in a limit in
which the target detection problem is ill-posed. Further analyzing quantum
illumination in a target-agnostic thermal environment shows that a weak
single-mode squeezed transmitter performs worse than ``no illumination'', which
is explained by the noise-increasing property of reflected low-intensity
squeezed light.
