If you've added any non-OEM electronics to the car, like audio or alarms, disconnecting them and seeing if it solves the problem is probably what I'd do first.
Someone here mentioned an idea a while ago, a sort of short-cut method, to find which circuit these current draws are in, that didn't require the fuses to be pulled. Any circuit drawing power will cause the fuse to heat up a little. They said they used a laser-guided non-contact temperature probe and pointed it at the fuses one by one. I haven't done it, but it seems like it would work. This gadget, it sounds like it would cost $500, but actually here's one that is quite inexpensive, less than $40.
Before using this method, you'd first want to measure the current at the battery to confirm you actually have a signficant phantom overall current draw. Anything over 100 mA when everything is off and sleeping would be suspicious. My Corolla, when I measured it, it had less than 10 mA with everything off.