Depending upon the test, it can be either a bad sector that can be (or is already) remapped, or it can problem sectors in other areas. For example, the SMART Extended Self Test can sometimes report all bad sectors even if they have already been remapped, while the Targeted Read Test will scan sectors near previously reported bad sectors to check for others.
If you look in the test logs, it should report where the errors are found so that you can focus additional testing on them as well as know approximately how many sectors are having problems.
Please let me know if this answers your question, and if you have any other problems.
Thanks,
Kris