This is tricky. Have the brake lights been replaced with LEDs? Sometimes there is a tiny current running that would not illuminate a traditional bulb but it would light an LED. I had this happen when I tried to upgrade the lighting in the boot of an old Fabia we have. Also, if there are just bulbs, check they do not have two filaments in.
check any wires you can see do not have damaged insulation allowing current to pass from one wire to another by accident.