Hard disk servo information is essential for the drive to operate. Without it the drive will be useless and probably won't even start up properly as the drive electronics would have a fit from the missing data.

It's more likely that the guy erased it or swapped the drive out. If it works and you've got the right amount of disk space then just ignore it

You would need an extremely powerful magnetic field to erase a 3.5" HD. The high data density on the platters mean you need a powerful field to rewrite the bits. X-ray machines could fog certain types of photographic film but that's about it.