All filters in a scanner gets connected in line with switching diodes. When the scanner are without power the diodes doesn't conduct and doesn't connect any of the filters in line between antenna connector and the receiver, it's a total isolation between antenna and receiver. That's why you should have your scanner turned off when not in use or when bad wheater are expected as no high energy spikes from a lightning strike nearby, or someone tranmitting close by, will reach the delicate electronics in the reciever.
/Ubbe