No ship (base, etc.) is truly "without engines" - that "1 kms" speed (station-keeping drive, manoeuvring thrusters, whatever) every non-PDC unit gets means it has a thermal signature of at least 1. Even a small-engin'd unit with high thermal reduction still has a thermal signature of at least 1. Switch your thermal sensors display to show you the range at which you can pick up a strength-1 thermal contact, and that will show you the range inside of which nothing can truly be 'invisible' - it's also far in excess of the range at which you could see something by 'looking out the window'.