So, I've discovered that there's a 2.57GB .dmg file in the /Users/username/firmware folder on my Mac (High Sierra v. 10.13.6). The filename is <058-97232-130.dmg>. There are a couple of other dmg files there too, closer to 80MB in size.
I was thinking that it's some kind of firmware update for my phone, except it's a .dmg file, not an ipsw. I've googled the filename and the folder location, and found nothing at all. Any ideas?
EDIT: Oh, one more weird thing: the file's creation/last modified date is "January/9/07" (which is the same date for the other .dmg files in the folder). But I got my computer in 2013, so I'm not sure what that tells us.