Thunderbird 52.0.1 "Search across all email accounts?"

Are you using Global search? https://support.mozilla.org/en-US/kb/global-search

SharkyCA said

When performing a search the old TB used to say "enter return to search everywhere" what happened to search?

My problem! Did not realize the search box above "message filter search" would do this. It used to offer the full search after the filter search.

I'd expect it to offer the continued search only if it has failed to find anything in the current folder. AFAIK it simply transfers what it can of your search specification to the global search engine.

I'm a bit unsure of the exact details because I use an add-on (Gmail UI/Expression Search) that enhances the QuickFilter search tool you have shown us.