Every HDD has a number of unused sectors that are preserved from the factory for the relocation events. Once HDD firmware detects unreadable sector it "replaces" it with the healthy one "from the stock". In fact, nothing is really moved, it only records that instead of sector xxx sector yyy must be used. This is called sector relocation event.
If it started to happen that means that HDD is not healthy, it's surface started degradation, the number of relocated sectors will increase in the future depending on how heavily HDD is used. So far, you can relax you can still work with this HDD, but you need to monitor the relocation progress and consider to replace HDD in the future.