It's not provisioned space from his SSD. That would show up as unallocated space on Disk0. He's not overprovisioning that SSD at all. Newer versions of Magician automatically reserve 10% of the disk for OP and you can vastly extend the life of the SSD by using 30% for OP instead. This situation does indeed seem to be mysterious if in fact the 29GB of phantom unallocated space came from an unreadable 32GB external drive that is no longer connected. Add to that the fact the space was able to be allocated and formatted? Mysterious... Mysterious indeed!
If it's a laptop, check the SD card slot. Had one retard moment in the past where I accidentally left a 32GB SD card in there and wondered where that drive came. Usually Windows does NOT assign drive letters for unformatted drives.