Handsearching important journals & reference tracking are ways of making sure that relevant articles do not fall through the cracks.
Handsearching (or browsing) journals is recommended because some journals are not indexed in databases.
Reference tracking can be done in low-tech (simply scanning reference lists of included articles) or high-tech (using citation tracking features on databases) ways.
You might consult a librarian (or your reporting standards) to see if these methods are suitable for your review. At least one study has concluded that reference tracking is not an effective use of a review team's time.
Below is a list of databases that provide reference tracking features: