Frustrating And Elusive Electrical Drain

I love my Base 2005 Prius. It has 250,000 miles on it, and aside from being a little hungrier for oil and gas than it used to be, it has had almost no problems, and only one I haven't been able to fix on my own (note: I am an amateur youtube mechanic at best). This issue, though, is driving me nuts, and it has me going through 12V batteries like AAAs. It started with what I considered to be a harmless ghost living in the radio, who likes to keep it on and listen for a little while after I turn off the car. The information display turns off and the controls stop working (besides the volume knob, which continues to function), but the sound continues for a non-specific amount time before suddenly stopping on its own. For a while I thought it was no big deal, as it always turned off eventually, but eventually I realized - usually via a dead battery at the end of a long shift - that even when the radio does turn off, there is still a power draw. I borrowed a friend's multi-meter as well as her electrical engineering background to try and diagnose it. We ripped apart the dash and I unplugged everything I could see, testing the drain with the multi-meter. There were lots of ~little~ drains - which isn't surprising given how long it's lived under the desert sun - but nothing big enough to account for the overall drain. If I unplug the fuses for the Radio / Information Display (I have already replaced the display once trying to fix this issue), or if I disconnect / reconnect the 12V battery, it stops draining (at least at a rate that I care about). Unfortunately, these fuses are connected to a great deal of things in the car, and I can't find the thing that matters. Intermittently, the apparently-classic "Audio Off" issue rears its head, the radio won't turn on at all, and I can breathe easy knowing the 12V battery is safe (for now). In between those periods of respite, I have to open the trunk and disconnect/reconnect the battery every time I leave the car. It's annoying, I'd like to fix it, and I haven't found anyone else either in person or online with a relatable experience, though I admit I may be barking up the wrong trees. If anyone has any advice, I'd love to hear it. Thanks.