KEY OFF BATTERY DRAIN

I've been trying to find the cause of a battery drain problem on my Volvo 850 (1997). The battery drains in about 3 days to the point that it won't start. In trying to assess the problem, I have pulled the fuses one at a time and measured the mV across the fuse contacts with the car off. This may not be the best way to this so maybe you can tell me the best way to do this, but I am getting very high voltage readings on several fuses and 10-20mV on most others. This is puzzling and if I'm doing this properly does that mean the ignition switch is not turning these devices off? For example the reading across the radio amplifier fuse contacts is 9V with the ignition off, yet the radio will not play with the ignition switch off.
Is it better to test by putting the multimeter in series to the battery and measure the voltage draw and then pull each fuse one at a time? Would really appreciate any help but as we all know, this is the kind of problem that creates grey hair.
Thanks.