The serial number may be held in eeprom while the toneprints are in flash memory.
This is a not untypical way of doing it, eeprom tends to stand far fewer rewrites and is slower to access and suits factory loaded data, like serial numbers, but is easier to protect from corruption when uploading new data (e.g. if a toneprint upload gets interrupted).
Unfortunately EEPROM is more liable to corruption from power spikes etc., I've had this happen on powering up chips on things I've made.
-1 suggests it is reading all ones for the serial number bits so if EEPROM it has been programmed (default is for eeprom to read all zeroes).
Alternatively the serial number may be written in a protected area of flash memory, this defaults to every bit at 1 which would suggest the serial has never been written, on reflection this may be more likely.