"Citation pearl growing" uses a relevant source to lead to more sources on a topic and can be used as an important complement to structured database searching for review projects.
When you have an article that exemplifies the type of literature you're trying to find (a sentinel article), you can pull up the record for that article in multiple databases and look for links to "related" articles or “find similar results." The database search platforms will analyze the metadata in the record and try to provide results that seem similar, based on their algorithms.
For any papers that have been identified for inclusion in your review, (or any review articles that are topically similar), you can mine the reference list to follow up on any citations that may also meet your inclusion criteria.
For any sentinel articles, or related review articles, you can perform 'cited reference searching'. This process tracks what articles have cited those relevant papers since the time when they were published, so it can be especially useful for older articles (they're likely to have been cited more than newer publications).
See this page for more information about the databases and search engines that allow you to perform cited reference searching.
As you become more familiar with the landscape of scholarly literature on your topic, you will begin to notice certain journals where relevant research is frequently published. Often, you can manually search into those journals directly (often into the full text of the articles) or skim their tables of content to try to identify any additional papers that may have slipped through the cracks.
Locating sources outside of the database searches is a common practice in systematic evidence synthesis to ensure that no relevant papers are unintentionally omitted from the review. As such, it is typical to report the number of 'Other Sources' in a systematic search flow diagram (PRISMA diagrams) separate from the database searches.
With that said, if most of the studies that were included in the synthesis came from 'other sources' (outside the structured database searches), that may be a red flag that the database search queries were not as broad (sensitive) as they needed to be to capture the relevant literature. If that is the case, it may be necessary to revise the database search strategies based on the set of articles you've already identified to include.