Bad sectors can be confusing.
When the PC requests data from the drive, but the drive is unable to give the data to the PC - it will tell the PC that it's come across a 'bad sector'. Generally, the OS will then make a record of that bad sector somewhere else on the drive. When the drive is next used, the OS will check the list of bad sectors and avoid them.
If you perform a 'quick format' Windows will preserve the list of bad sectors. However, if you 'zero-fill' the drive, or format it as a different format (e.g. in linux) then the list of bad sectors will be lost. Normally, during a 'full' format, the OS will try and read the entire drive, that way if there are any bad sectors, they will be detected and the list regenerated.
However, not all sectors are bad all the time (sometimes the drive gets lucky and can read it) - this means that a drive reformatted after 'zero-filling' may not have a complete list of bad sectors after formatting.
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Some people have claimed to have 'repaired' bad sectors. What's going on here?
All hard drives will have some bad sectors when they come from the factory. They can't be made perfect enough. So how come, when you get a brand new drive, it's perfect in scandisk? The answer is that drives contain a number of 'spare' sectors. During normal use the drive will calculate quality scores for each sector as it's accessed, and periodically scan the entire drive. If a sector is going bad, then it will automatically copy the data into a spare sector, and disable the original sector. Or, if the data is bad and unrecoverable, it will disable that sector and activate a spare.
This dealing with spare sectors is done automatically by the drive. The OS has no access or control over it.
So, sometimes if a sector suddenly goes bad - scandisk will report it as bad. But at the same time, the drive will replace it with a spare. Scandisk has no way of knowing that a spare has been recruited, so it carries on as normal. However, if you zero-fill the drive and reformat, the bad sector will have gone (it will appear ot have been repaired). This is relatively uncommon.
What happens more commonly, is that spares are recruited to replacing weakening sectors. Eventually, all the spares get used up - and the next time a sector dies - the drive has no choice but to call it bad. In general, if you've got more than 1 bad sector on a drive, this is what has happened. Essentially, the drive has suffered critical damage which has used up all the spares. In general, such a drive is likely to be very unreliable and have a very short life. It should be replaced immediately.
