A lot of older Volvos seem to have the window switches miswired (from shop, not factory), and I really don't know anything about the wiring in this car but maybe the wiring is confusing and someone mis-wired it to the always-on line or ignition-off line or something. Speaking from an electrical point-of-view, I would open up the door panel and check the voltage between the high side of each of the switches and relays and ground with the ignition in each of its positions. If the rear windows have 12V with the ignition off and 0V with it off, it's probably miswiring. If it's something else, like 12V off and less than 12V on, or less than 12V off it could be something awful like a wire touching the metal of the body somewhere.

Hope that helps,
Richard