Indie404 asks whether your 404 is actually a 404

Field notes

The checker takes a domain or home URL, creates a generated missing path, and evaluates the resulting HTTP response rather than stopping at the status code. Its result layout separates redirects, final URL, page titles, visible-text overlap, indexing signals, and response source; it also identifies RFC 9110 and Google Search Central as references. The page states that checks do not become public listings and that no lookup history or request body is stored. The extracted view remained at the waiting state, so completed results were not observed.