`Find_source_in_db` returns a single element list

It's not ideal that `find_source_in_db` returns a single element list. Ideally, it would return a string. I see how that could make error handeling a little harder (since the function will return either a string or a list), but worth thinking about how to improve.

Locate the find_source_in_db function and inspect its callers to understand how the current single-element list is consumed, especially in error handling. Clarify the intended return contract and review any existing tests or usage examples before changing it; done means the behavior is consistent for all relevant lookup outcomes and callers handle it correctly.

Written by the indexing model from the issue text.