OK, I think I understand now. While I haven't ever re-connected my negative battery cable to its post at any time during my procedure, what I've been doing amounts to the same thing. I've been leaving the cable disconnected from its post, but then connecting a 1 ohm resistor in series between the cable and its post, and then measuring the voltage drop across the resistor. The problem has been that I had to disconnect the resistor after each test, because the current draw is so huge that it generates 16 watts (I-squared-R, where I = 4 amps, and R = 1 ohm), and the resistor is only rated at 10 watts, so if I had kept it connected long enough for everything to go to sleep, it would have burned up the resistor. This meant that I had to disconnect and re-connect the resistor every time I checked a fuse, so I inadvertently "woke up" the systems every time I tested a fuse.

So, here's what I just did:

- Removed the connectors to both of the rear hatch interior lights
- Pulled fuse 27
- Closed all the doors
- Connected the 1 ohm resistor in series between the negative battery terminal and the cable
- Measured the voltage drop across the resistor to be 0.4 volts, which equates to 400 mA
- Waited about 45 minutes
- Checked the voltage again - it had dropped to 0.038 volts, or right at the target 38 mA

So, I've established that my current drain is located somewhere in the circuits protected by fuse 27, and nowhere else (I've checked every single fuse in the vehicle, including the three that protect the blower, ruling out the FSR). Unfortunately, this one fuse, according to the label in the glove box, protects three separate circuits (glove box light, passenger compartment/trunk lights, and windshield washer system). I've ruled out the windshield washer pump by disconnecting its connector, so maybe it's one of the lights (other than the hatch lights, because I've already determined that I get the same 4+ amps flow with the hatch light connectors disconnected if I keep fuse 27 in place).

Thanks again for all the advice - I'm definitely learning a lot about how to troubleshoot this type of problem. Any tips on how to track down the actual source of the problem? I guess I'll start by disconnecting the glove box light.

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2003 X5 4.4i
1992 Porsche 968
2007 328i
2014 Subaru Forester XT