. . it was still there immediately after a reinstall, and still there immediately after a format - reinstall, but no trace after a security wipe of 100% of the disk indicates to me it was not in the BIOS.

Boot kits are very clever, and apparently now even more clever. If something looks at the boot sector, what it sees is a fake one that looks normal.

When Windows partitions a disk, it always leaves 8 Megs of free space at the end of the disk. This is an open invitation to the bad guys to use that space, setting up a hidden partition.