It was on a bookshelf in the family room, on top of a CD-R drive and a 5.25" hard drive, and under my chuu-Totoro plush. It was in back, behind some other things, and not immediately obvious. WTFF. I have no idea why it was put there, though I'm pretty sure it's been there since about January of 2013, when Mrs. Fungus moved in here.
Mrs. Fungus called me into the family room to ask me a couple of questions, and while I was answering them I looked at the bookshelf. My eyes fell on the text and for a few moments I didn't understand the words; finally I realized what "SATA/IDE TO USB 2.0 Adapter" meant and realized I'd finally found the f-ing thing.
It was quite literally in the last place I'd look, or even think to look. *sigh*
The 5.25" hard drive is a 4 GB drive and it has a Compaq part number on it, so I'm not sure whether it's the HDD from Mom's computer or what. Fortunately, since I now have the HDD interface, I can check it. It's a party!
In related news, I've discovered that the signal cable that connects my speakers to the computer has died. If I kick the cable, the speakers cut out. That means I need either to get new speakers, or to repair these. "Repair" means getting the soldering iron out, which shouldn't be too difficult...and if it turns out that these aren't repairable, well, I have three or four complete sets of computer speakers I can use in their stead. WTF.
It is kind of a pain, though.